[libvirt] [RFC v2 REPOST] arm64: KVM: KVM API extensions for SVE
Hi all, Reposting this to give people another chance to comment before I move ahead... ---8<--- Here's a second, slightly more complete stab at the KVM API extensions for SVE. I haven't started implementing in earnest yet, so any comments at this stage would be very helpful. [libvir-list readers: this is a proposal for extending the KVM API on AArch64 systems to support the Scalable Vector Extension [1], [2]. This has some interesting configuration and migration quirks -- see "Vector length control" in particular, and feel free to throw questions my way...] Cheers ---Dave [1] Overview https://community.arm.com/processors/b/blog/posts/technology-update-the-scal... [2] Architecture spec https://developer.arm.com/products/architecture/a-profile/docs/ddi0584/lates... ---8<--- New feature KVM_ARM_VCPU_SVE: * enables exposure of SVE to the guest * enables visibility of / access to KVM_REG_ARM_SVE_*() via KVM reg ioctls. The main purposes of this are a) is to allow userspace to hide weird-sized registers that it doesn't understand how to deal with, and b) allow SVE to be hidden from the VM so that it can migrate to nodes that don't support SVE. ZCR_EL1 is not specifically hidden, since it is "just a system register" and does not have a weird size or semantics etc. Registers: * A new register size is defined KVM_REG_SIZE_U2048 (which can be encoded sensibly using the next unused value for the reg size field in the reg ID) (grep KVM_REG_SIZE_). * Reg IDs for the SVE regs will be defined as "coproc" 0x14 (i.e., 0x14 << KVM_REG_ARM_COPROC_SHIFT) KVM_REG_ARM_SVE_Z(n, i) is slice i of Zn (each slice is 2048 bits) KVM_REG_ARM_SVE_P(n, i) is slice i of Pn (each slice is 256 bits) KVM_REG_ARM_FFR(i) is slice i of FFR (each slice is 256 bits) The slice sizes allow each register to be read/written in exactly one slice for SVE. Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore. Reading/writing surplus slices will probably be forbidden, and the surplus slices would not be reported via KVM_GET_REG_LIST. (We could make these RAZ/WI too, but I'm not sure if it's worth it, or why it would be useful.) Future extensions to the architecture might grow the registers up to 32 slices: this may or may not actually happen, but SVE keeps the possibilty open. I've tried to design for it. * KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3. It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet. Vector length control: Some means is needed to determine the set of vector lengths visible to guest software running on a vcpu. When a vcpu is created, the set would be defaulted to the maximal set that can be supported while permitting each vcpu to run on any host CPU. SVE has some virtualisation quirks which mean that this set may exclude some vector lengths that are available for host userspace applications. The common case should be that the sets are the same however. * New ioctl KVM_ARM_VCPU_{SET,GET}_SVE_VLS to set or retrieve the set of vector lengths available to the guest. Adding random vcpu ioctls To configure a non-default set of vector lengths, KVM_ARM_VCPU_SET_SVE_VLS can be called: this would only be permitted before the vcpu is first run. This is primarily intended for supporting migration, by providing a robust check that the destination node will run the vcpu correctly. In a cluster with non-uniform SVE implementation across nodes, this also allows a specific set of VLs to be requested that the caller knows is usable across the whole cluster. For migration purposes, userspace would need to do KVM_ARM_VCPU_GET_SVE_VLS at the origin node and store the returned set as VM metadata: on the destination node, KVM_ARM_VCPU_SET_SVE_VLS should be used to request that exact set of VLs: if the destination node can't support that set of VLs, the call will fail. The interface would look something like: ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]); How to expose this to the user in an intelligible way would be a problem for userspace to solve. At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails. Cheers ---Dave _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
Hi Dave, On Fri, Jan 26, 2018 at 05:28:49PM +0000, Dave Martin wrote:
New feature KVM_ARM_VCPU_SVE:
* enables exposure of SVE to the guest
* enables visibility of / access to KVM_REG_ARM_SVE_*() via KVM reg ioctls. The main purposes of this are a) is to allow userspace to hide weird-sized registers that it doesn't understand how to deal with, and b) allow SVE to be hidden from the VM so that it can migrate to nodes that don't support SVE.
ZCR_EL1 is not specifically hidden, since it is "just a system register" and does not have a weird size or semantics etc.
I think you want to hide ZCR_EL1 if SVE is not enabled, since presenting it to a legacy userspace will prevent migration onto a non-SVE enabled kernel/machine.
Registers:
* A new register size is defined KVM_REG_SIZE_U2048 (which can be encoded sensibly using the next unused value for the reg size field in the reg ID) (grep KVM_REG_SIZE_).
* Reg IDs for the SVE regs will be defined as "coproc" 0x14 (i.e., 0x14 << KVM_REG_ARM_COPROC_SHIFT)
KVM_REG_ARM_SVE_Z(n, i) is slice i of Zn (each slice is 2048 bits) KVM_REG_ARM_SVE_P(n, i) is slice i of Pn (each slice is 256 bits) KVM_REG_ARM_FFR(i) is slice i of FFR (each slice is 256 bits)
Just so I understand; slice 0 covers all the SVE state as the SVE spec is today. Additional slices is something we can use in the future in case the SVE spec is expanded to allow even larger vector lengths? In which case, for a hypothetical 4096 bits register, slice 0 of Zn will be the lower 2048 bits and slice 1 will be the upper 2048 bits?
The slice sizes allow each register to be read/written in exactly one slice for SVE.
Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore.
This implies that there are some ordering fun to be had between accessing SVE registers and changing the maximum vector length for the vcpu. Why not loosen the API slightly and say that anything written to the surplus bits is not guaranteed to be preserved when read back? (In other words, we make no guarantees).
Reading/writing surplus slices will probably be forbidden, and the surplus slices would not be reported via KVM_GET_REG_LIST. (We could make these RAZ/WI too, but I'm not sure if it's worth it, or why it would be useful.)
Future extensions to the architecture might grow the registers up to 32 slices: this may or may not actually happen, but SVE keeps the possibilty open. I've tried to design for it.
* KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3.
nit: KVM_REG_ARM_CORE_REG doesn't specify the size, so I think this would be more clearly stated as "aliases KVM_REG_SIZE_U128 | KVM_REG_ARM_CORE(fp_regs.vregs[n])" or as "aliases kvm_regs.regs.fp_regs.vregs[n]".
It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet.
It's not going to be a big deal, and it's the only sensible thing to do; otherwise we'll have no clear semantics of where the values are. If KVM chooses to duplicate the 128 bits of state in memory, then it must also know how to syncrhonize that duplicated state when running the guest, and therefore can also do this in software on SET/GET.
Vector length control:
Some means is needed to determine the set of vector lengths visible to guest software running on a vcpu.
When a vcpu is created, the set would be defaulted to the maximal set that can be supported while permitting each vcpu to run on any host CPU. SVE has some virtualisation quirks which mean that this set may exclude some vector lengths that are available for host userspace applications. The common case should be that the sets are the same however.
* New ioctl KVM_ARM_VCPU_{SET,GET}_SVE_VLS to set or retrieve the set of vector lengths available to the guest.
Adding random vcpu ioctls
To configure a non-default set of vector lengths, KVM_ARM_VCPU_SET_SVE_VLS can be called: this would only be permitted before the vcpu is first run.
This is primarily intended for supporting migration, by providing a robust check that the destination node will run the vcpu correctly. In a cluster with non-uniform SVE implementation across nodes, this also allows a specific set of VLs to be requested that the caller knows is usable across the whole cluster.
For migration purposes, userspace would need to do KVM_ARM_VCPU_GET_SVE_VLS at the origin node and store the returned set as VM metadata: on the destination node, KVM_ARM_VCPU_SET_SVE_VLS should be used to request that exact set of VLs: if the destination node can't support that set of VLs, the call will fail.
The interface would look something like:
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]);
I don't understand the proposed semantics here. What is the vqs array and how should the values be set?
How to expose this to the user in an intelligible way would be a problem for userspace to solve.
That's fine.
At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails.
I think we should have such a way. Libvirt can do things like figure out compatible features across nodes, and trying to set all possible vector lengths doesn't seem like a great approach. How about creating the interface based on a bitmap of all the possible lengths, and setting the bits for all lengths, and reading back the value returns the actual supported lengths? My comments above are really in the details so I think you can safely start putting together a full implementation, assuming this is still your plan. Thanks, -Christoffer
On Mon, Feb 05, 2018 at 05:13:08PM +0100, Christoffer Dall wrote:
Hi Dave,
On Fri, Jan 26, 2018 at 05:28:49PM +0000, Dave Martin wrote:
New feature KVM_ARM_VCPU_SVE:
* enables exposure of SVE to the guest
* enables visibility of / access to KVM_REG_ARM_SVE_*() via KVM reg ioctls. The main purposes of this are a) is to allow userspace to hide weird-sized registers that it doesn't understand how to deal with, and b) allow SVE to be hidden from the VM so that it can migrate to nodes that don't support SVE.
ZCR_EL1 is not specifically hidden, since it is "just a system register" and does not have a weird size or semantics etc.
I think you want to hide ZCR_EL1 if SVE is not enabled, since presenting it to a legacy userspace will prevent migration onto a non-SVE enabled kernel/machine.
On reflection, I think you're right here. It makes no sense to half- expose SVE.
Registers:
* A new register size is defined KVM_REG_SIZE_U2048 (which can be encoded sensibly using the next unused value for the reg size field in the reg ID) (grep KVM_REG_SIZE_).
* Reg IDs for the SVE regs will be defined as "coproc" 0x14 (i.e., 0x14 << KVM_REG_ARM_COPROC_SHIFT)
KVM_REG_ARM_SVE_Z(n, i) is slice i of Zn (each slice is 2048 bits) KVM_REG_ARM_SVE_P(n, i) is slice i of Pn (each slice is 256 bits) KVM_REG_ARM_FFR(i) is slice i of FFR (each slice is 256 bits)
Just so I understand; slice 0 covers all the SVE state as the SVE spec is today. Additional slices is something we can use in the future in case the SVE spec is expanded to allow even larger vector lengths? In which case, for a hypothetical 4096 bits register, slice 0 of Zn will be the lower 2048 bits and slice 1 will be the upper 2048 bits?
Yes. The architecture leaves the decision about future expansion open. It may never happen, but I'm aiming for a design that will accommodate it if it does happen. Exposing variable-size regs directly through the ioctl interface, or growing the ioctl view of the regs much beyond 2048 bits seems impractical, so the sliced view seemed reasonable. In practice, we might never need more than one slice anyway.
The slice sizes allow each register to be read/written in exactly one slice for SVE.
Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore.
This implies that there are some ordering fun to be had between accessing SVE registers and changing the maximum vector length for the vcpu. Why not loosen the API slightly and say that anything written to the surplus bits is not guaranteed to be preserved when read back? (In other words, we make no guarantees).
I'm happy enough to say that, since it may remove some burden from the implementation; though only memzeroing so perhaps not too exciting.
Reading/writing surplus slices will probably be forbidden, and the surplus slices would not be reported via KVM_GET_REG_LIST. (We could make these RAZ/WI too, but I'm not sure if it's worth it, or why it would be useful.)
Future extensions to the architecture might grow the registers up to 32 slices: this may or may not actually happen, but SVE keeps the possibilty open. I've tried to design for it.
* KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3.
nit: KVM_REG_ARM_CORE_REG doesn't specify the size, so I think this would be more clearly stated as "aliases KVM_REG_SIZE_U128 | KVM_REG_ARM_CORE(fp_regs.vregs[n])" or as "aliases kvm_regs.regs.fp_regs.vregs[n]".
I thought the size was effectively hard-coded to be 64 bits for KVM_REG_ARM_CORE(). Or did I misunderstand?
It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet.
It's not going to be a big deal, and it's the only sensible thing to do; otherwise we'll have no clear semantics of where the values are. If KVM chooses to duplicate the 128 bits of state in memory, then it must also know how to syncrhonize that duplicated state when running the guest, and therefore can also do this in software on SET/GET.
I tried to remember my full rationale and describe what the rules would be if the two views are allowed to be incoherent ... and concluded that you are probably correct. This also matches the architectural view of the registers, which is what users would naturally expect. Software intentionally modifying the registers would need to know about this aliasing, but since software is explicitly required to enable the KVM_ARM_VCPU_SVE feature, this shouldn't pose problems for backwards compatibility.
Vector length control:
Some means is needed to determine the set of vector lengths visible to guest software running on a vcpu.
When a vcpu is created, the set would be defaulted to the maximal set that can be supported while permitting each vcpu to run on any host CPU. SVE has some virtualisation quirks which mean that this set may exclude some vector lengths that are available for host userspace applications. The common case should be that the sets are the same however.
* New ioctl KVM_ARM_VCPU_{SET,GET}_SVE_VLS to set or retrieve the set of vector lengths available to the guest.
Adding random vcpu ioctls
To configure a non-default set of vector lengths, KVM_ARM_VCPU_SET_SVE_VLS can be called: this would only be permitted before the vcpu is first run.
This is primarily intended for supporting migration, by providing a robust check that the destination node will run the vcpu correctly. In a cluster with non-uniform SVE implementation across nodes, this also allows a specific set of VLs to be requested that the caller knows is usable across the whole cluster.
For migration purposes, userspace would need to do KVM_ARM_VCPU_GET_SVE_VLS at the origin node and store the returned set as VM metadata: on the destination node, KVM_ARM_VCPU_SET_SVE_VLS should be used to request that exact set of VLs: if the destination node can't support that set of VLs, the call will fail.
The interface would look something like:
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]);
I don't understand the proposed semantics here. What is the vqs array and how should the values be set?
vqs is a bitmap, where bit (n - 1) indicates support for VL = 128n bits. Since the hardware is not required to support all possible vector lengths up to the maximum, a bitmap seemed the most straightforward way of representing the possibly sparse set of supported VLs. Calling it vq_bitmap[] would make this clearer. Describing only the maximum supported vl turns out to be insufficient. [...]
At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails.
I think we should have such a way. Libvirt can do things like figure out compatible features across nodes, and trying to set all possible vector lengths doesn't seem like a great approach.
How about creating the interface based on a bitmap of all the possible lengths, and setting the bits for all lengths, and reading back the value returns the actual supported lengths?
Would it be sufficient for an initial call to KVM_ARM_VCPU_GET_SVE_VLS to return the default (i.e., maximum supported) set? This would mean that once KVM_ARM_VCPU_SET_SVE_VLS has been called to set a different set, it would no longer be possible to find out the maximum set. I'm not sure this would matter, though.
My comments above are really in the details so I think you can safely start putting together a full implementation, assuming this is still your plan.
Absolutely... and thanks for the feedback. Cheers ---Dave
On Tue, Feb 06, 2018 at 11:43:16AM +0000, Dave Martin wrote:
On Mon, Feb 05, 2018 at 05:13:08PM +0100, Christoffer Dall wrote:
Hi Dave,
On Fri, Jan 26, 2018 at 05:28:49PM +0000, Dave Martin wrote:
New feature KVM_ARM_VCPU_SVE:
* enables exposure of SVE to the guest
* enables visibility of / access to KVM_REG_ARM_SVE_*() via KVM reg ioctls. The main purposes of this are a) is to allow userspace to hide weird-sized registers that it doesn't understand how to deal with, and b) allow SVE to be hidden from the VM so that it can migrate to nodes that don't support SVE.
ZCR_EL1 is not specifically hidden, since it is "just a system register" and does not have a weird size or semantics etc.
I think you want to hide ZCR_EL1 if SVE is not enabled, since presenting it to a legacy userspace will prevent migration onto a non-SVE enabled kernel/machine.
On reflection, I think you're right here. It makes no sense to half- expose SVE.
Registers:
* A new register size is defined KVM_REG_SIZE_U2048 (which can be encoded sensibly using the next unused value for the reg size field in the reg ID) (grep KVM_REG_SIZE_).
* Reg IDs for the SVE regs will be defined as "coproc" 0x14 (i.e., 0x14 << KVM_REG_ARM_COPROC_SHIFT)
KVM_REG_ARM_SVE_Z(n, i) is slice i of Zn (each slice is 2048 bits) KVM_REG_ARM_SVE_P(n, i) is slice i of Pn (each slice is 256 bits) KVM_REG_ARM_FFR(i) is slice i of FFR (each slice is 256 bits)
Just so I understand; slice 0 covers all the SVE state as the SVE spec is today. Additional slices is something we can use in the future in case the SVE spec is expanded to allow even larger vector lengths? In which case, for a hypothetical 4096 bits register, slice 0 of Zn will be the lower 2048 bits and slice 1 will be the upper 2048 bits?
Yes. The architecture leaves the decision about future expansion open. It may never happen, but I'm aiming for a design that will accommodate it if it does happen.
Exposing variable-size regs directly through the ioctl interface, or growing the ioctl view of the regs much beyond 2048 bits seems impractical, so the sliced view seemed reasonable. In practice, we might never need more than one slice anyway.
The slice sizes allow each register to be read/written in exactly one slice for SVE.
Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore.
This implies that there are some ordering fun to be had between accessing SVE registers and changing the maximum vector length for the vcpu. Why not loosen the API slightly and say that anything written to the surplus bits is not guaranteed to be preserved when read back? (In other words, we make no guarantees).
I'm happy enough to say that, since it may remove some burden from the implementation; though only memzeroing so perhaps not too exciting.
Reading/writing surplus slices will probably be forbidden, and the surplus slices would not be reported via KVM_GET_REG_LIST. (We could make these RAZ/WI too, but I'm not sure if it's worth it, or why it would be useful.)
Future extensions to the architecture might grow the registers up to 32 slices: this may or may not actually happen, but SVE keeps the possibilty open. I've tried to design for it.
* KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3.
nit: KVM_REG_ARM_CORE_REG doesn't specify the size, so I think this would be more clearly stated as "aliases KVM_REG_SIZE_U128 | KVM_REG_ARM_CORE(fp_regs.vregs[n])" or as "aliases kvm_regs.regs.fp_regs.vregs[n]".
I thought the size was effectively hard-coded to be 64 bits for KVM_REG_ARM_CORE(). Or did I misunderstand?
It's not. KVM_REG_ARM_CORE only encodes the 'group' of registers. Userspace then has to OR on the size. So the 128 bits FP regs are accessed with the following logic in QEMU: #define AARCH64_SIMD_CORE_REG(x) \ (KVM_REG_ARM64 | KVM_REG_SIZE_U128 | \ KVM_REG_ARM_CORE | KVM_REG_ARM_CORE_REG(x)) reg.id = AARCH64_SIMD_CORE_REG(fp_regs.vregs[i]); Looking back, the macros in kvm.h are potentially a bit silly, and should rather have a full AARCH64_SIMD_CORE_REG() definition (maybe we should add that), but the idea is to use the offset into the kvm structure as an indication for *which* register to retrieve, and then encode the size of the register at the same time, and provide a pointer to a value of the same size, which should avoid any endianness problems etc.
It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet.
It's not going to be a big deal, and it's the only sensible thing to do; otherwise we'll have no clear semantics of where the values are. If KVM chooses to duplicate the 128 bits of state in memory, then it must also know how to syncrhonize that duplicated state when running the guest, and therefore can also do this in software on SET/GET.
I tried to remember my full rationale and describe what the rules would be if the two views are allowed to be incoherent ... and concluded that you are probably correct. This also matches the architectural view of the registers, which is what users would naturally expect.
Some early experiences taught us that anything we expose to user space should really be as close to the architectural concepts as possible, otherwise we quickly venture into land of ambiguity, which is a terrible place to be for a kernel<->userspace ABI.
Software intentionally modifying the registers would need to know about this aliasing, but since software is explicitly required to enable the KVM_ARM_VCPU_SVE feature, this shouldn't pose problems for backwards compatibility.
Agreed.
Vector length control:
Some means is needed to determine the set of vector lengths visible to guest software running on a vcpu.
When a vcpu is created, the set would be defaulted to the maximal set that can be supported while permitting each vcpu to run on any host CPU. SVE has some virtualisation quirks which mean that this set may exclude some vector lengths that are available for host userspace applications. The common case should be that the sets are the same however.
* New ioctl KVM_ARM_VCPU_{SET,GET}_SVE_VLS to set or retrieve the set of vector lengths available to the guest.
Adding random vcpu ioctls
To configure a non-default set of vector lengths, KVM_ARM_VCPU_SET_SVE_VLS can be called: this would only be permitted before the vcpu is first run.
This is primarily intended for supporting migration, by providing a robust check that the destination node will run the vcpu correctly. In a cluster with non-uniform SVE implementation across nodes, this also allows a specific set of VLs to be requested that the caller knows is usable across the whole cluster.
For migration purposes, userspace would need to do KVM_ARM_VCPU_GET_SVE_VLS at the origin node and store the returned set as VM metadata: on the destination node, KVM_ARM_VCPU_SET_SVE_VLS should be used to request that exact set of VLs: if the destination node can't support that set of VLs, the call will fail.
The interface would look something like:
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]);
I don't understand the proposed semantics here. What is the vqs array and how should the values be set?
vqs is a bitmap, where bit (n - 1) indicates support for VL = 128n bits.
Since the hardware is not required to support all possible vector lengths up to the maximum, a bitmap seemed the most straightforward way of representing the possibly sparse set of supported VLs.
Calling it vq_bitmap[] would make this clearer.
Either way, I just think the details need to be spelled out.
Describing only the maximum supported vl turns out to be insufficient.
[...]
At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails.
I think we should have such a way. Libvirt can do things like figure out compatible features across nodes, and trying to set all possible vector lengths doesn't seem like a great approach.
How about creating the interface based on a bitmap of all the possible lengths, and setting the bits for all lengths, and reading back the value returns the actual supported lengths?
Would it be sufficient for an initial call to KVM_ARM_VCPU_GET_SVE_VLS to return the default (i.e., maximum supported) set?
This would mean that once KVM_ARM_VCPU_SET_SVE_VLS has been called to set a different set, it would no longer be possible to find out the maximum set. I'm not sure this would matter, though.
I don't particularly like this, because the init sequence in QEMU can be pretty complicated, and I think it can become pretty nasty to have to remember if we 'once upon a time' called some ioctl, because then calling it now will mean something else. In case my suggestion wasn't clear, this is what I meant: #define NR_VQS ((32 * 2048 / 128) / 64) __u64 vqs[NR_VQS] = { [0 ... NR_VQS - 1] = ~0 }; ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs); ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs); for (i = 0; i < 16; i++) if (vqs[0] & BIT(i)) printf("vector length %d supported by KVM\n", (i+1) * 128); Thanks, -Christoffer
On Tue, Feb 06, 2018 at 02:17:57PM +0100, Christoffer Dall wrote:
On Tue, Feb 06, 2018 at 11:43:16AM +0000, Dave Martin wrote:
On Mon, Feb 05, 2018 at 05:13:08PM +0100, Christoffer Dall wrote:
Hi Dave,
On Fri, Jan 26, 2018 at 05:28:49PM +0000, Dave Martin wrote:
[...]
Exposing variable-size regs directly through the ioctl interface, or growing the ioctl view of the regs much beyond 2048 bits seems impractical, so the sliced view seemed reasonable. In practice, we might never need more than one slice anyway.
The slice sizes allow each register to be read/written in exactly one slice for SVE.
Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore.
This implies that there are some ordering fun to be had between accessing SVE registers and changing the maximum vector length for the vcpu. Why not loosen the API slightly and say that anything written to the surplus bits is not guaranteed to be preserved when read back? (In other words, we make no guarantees).
I'm happy enough to say that, since it may remove some burden from the implementation; though only memzeroing so perhaps not too exciting.
[...]
* KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3.
nit: KVM_REG_ARM_CORE_REG doesn't specify the size, so I think this would be more clearly stated as "aliases KVM_REG_SIZE_U128 | KVM_REG_ARM_CORE(fp_regs.vregs[n])" or as "aliases kvm_regs.regs.fp_regs.vregs[n]".
I thought the size was effectively hard-coded to be 64 bits for KVM_REG_ARM_CORE(). Or did I misunderstand?
It's not. KVM_REG_ARM_CORE only encodes the 'group' of registers. Userspace then has to OR on the size. So the 128 bits FP regs are accessed with the following logic in QEMU:
#define AARCH64_SIMD_CORE_REG(x) \ (KVM_REG_ARM64 | KVM_REG_SIZE_U128 | \ KVM_REG_ARM_CORE | KVM_REG_ARM_CORE_REG(x))
reg.id = AARCH64_SIMD_CORE_REG(fp_regs.vregs[i]);
Looking back, the macros in kvm.h are potentially a bit silly, and should rather have a full AARCH64_SIMD_CORE_REG() definition (maybe we should add that), but the idea is to use the offset into the kvm structure as an indication for *which* register to retrieve, and then encode the size of the register at the same time, and provide a pointer to a value of the same size, which should avoid any endianness problems etc.
After another look at arch/arm64/kvm/guest.c:get_core_reg(), I get this now.
It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet.
It's not going to be a big deal, and it's the only sensible thing to do; otherwise we'll have no clear semantics of where the values are. If KVM chooses to duplicate the 128 bits of state in memory, then it must also know how to syncrhonize that duplicated state when running the guest, and therefore can also do this in software on SET/GET.
I tried to remember my full rationale and describe what the rules would be if the two views are allowed to be incoherent ... and concluded that you are probably correct. This also matches the architectural view of the registers, which is what users would naturally expect.
Some early experiences taught us that anything we expose to user space should really be as close to the architectural concepts as possible, otherwise we quickly venture into land of ambiguity, which is a terrible place to be for a kernel<->userspace ABI.
Agreed!
Software intentionally modifying the registers would need to know about this aliasing, but since software is explicitly required to enable the KVM_ARM_VCPU_SVE feature, this shouldn't pose problems for backwards compatibility.
Agreed.
Vector length control:
[...]
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]);
I don't understand the proposed semantics here. What is the vqs array and how should the values be set?
vqs is a bitmap, where bit (n - 1) indicates support for VL = 128n bits.
Since the hardware is not required to support all possible vector lengths up to the maximum, a bitmap seemed the most straightforward way of representing the possibly sparse set of supported VLs.
Calling it vq_bitmap[] would make this clearer.
Either way, I just think the details need to be spelled out.
Sure. This won't be the final documentation, but I'll bear it in mind. The vq bitmap concept is the same as used to track the supported vector lengths in fpsimd.c, but sane people are probably not aware of that.
Describing only the maximum supported vl turns out to be insufficient.
[...]
At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails.
I think we should have such a way. Libvirt can do things like figure out compatible features across nodes, and trying to set all possible vector lengths doesn't seem like a great approach.
How about creating the interface based on a bitmap of all the possible lengths, and setting the bits for all lengths, and reading back the value returns the actual supported lengths?
Would it be sufficient for an initial call to KVM_ARM_VCPU_GET_SVE_VLS to return the default (i.e., maximum supported) set?
This would mean that once KVM_ARM_VCPU_SET_SVE_VLS has been called to set a different set, it would no longer be possible to find out the maximum set. I'm not sure this would matter, though.
I don't particularly like this, because the init sequence in QEMU can be pretty complicated, and I think it can become pretty nasty to have to remember if we 'once upon a time' called some ioctl, because then calling it now will mean something else.
In case my suggestion wasn't clear, this is what I meant:
#define NR_VQS ((32 * 2048 / 128) / 64)
__u64 vqs[NR_VQS] = { [0 ... NR_VQS - 1] = ~0 };
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs); ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
for (i = 0; i < 16; i++) if (vqs[0] & BIT(i)) printf("vector length %d supported by KVM\n", (i+1) * 128);
This does mean that if the caller doesn't get the set they asked for in KVM_ARM_SVE_SET_VLS this is not treated as an error by the kernel. I was concerned this would encourage userspace to silently miss this error. What if KVM_ARM_SVE_SET_VLS() were to yield 0 if the exact requested set of VLs was configured, -ERANGE if some subset was configured successfully (but not exactly the set requested), and the usual -EINVAL/-EIO/whatever if the set of VLs couldn't be configured at all? Then the probe would go like this: __u64 vqs[SVE_VQ_MAX / 64] = { [0 ... SVE_VQ_MAX / 64 - 1] = ~(u64)0 }; if (ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs) && errno != ERANGE)) goto error; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs); /* ... */ Another option would be for SVE_ARM_SVE_SET_VLS to write the resulting set back to its argument, possibly with the same 0/ERANGE/EINVAL semantics. Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS, and check the resulting set: /* ... */ __u64 newvqs[SVE_VQ_MAX / 64]; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs); if (memcmp(vqs, newvqs, sizeof vqs)) goto mismatch; vcpu restore would need to treat any mismatch as an error: the exact requested set but be configurable, or the VM may go wrong. Any opinion on which approach is best? I was fighting with this in December, trying to come up with a way for userspace to specify which VLs it requires/permits/forbids and letting the kernel come up with something matching these constraints. But this seemed too expressive and complex, so I had dropped the idea in favour of something simpler. Cheers ---Dave
On Wed, Feb 07, 2018 at 11:33:28AM +0000, Dave Martin wrote:
On Tue, Feb 06, 2018 at 02:17:57PM +0100, Christoffer Dall wrote:
On Tue, Feb 06, 2018 at 11:43:16AM +0000, Dave Martin wrote:
On Mon, Feb 05, 2018 at 05:13:08PM +0100, Christoffer Dall wrote:
Hi Dave,
On Fri, Jan 26, 2018 at 05:28:49PM +0000, Dave Martin wrote:
[...]
Exposing variable-size regs directly through the ioctl interface, or growing the ioctl view of the regs much beyond 2048 bits seems impractical, so the sliced view seemed reasonable. In practice, we might never need more than one slice anyway.
The slice sizes allow each register to be read/written in exactly one slice for SVE.
Surplus bits (beyond the maximum VL supported by the vcpu) will be read-as-zero write-ignore.
This implies that there are some ordering fun to be had between accessing SVE registers and changing the maximum vector length for the vcpu. Why not loosen the API slightly and say that anything written to the surplus bits is not guaranteed to be preserved when read back? (In other words, we make no guarantees).
I'm happy enough to say that, since it may remove some burden from the implementation; though only memzeroing so perhaps not too exciting.
[...]
* KVM_REG_ARM_SVE_Z(n, 0) bits [127:0] alias Vn in KVM_REG_ARM_CORE(fp_regs.v[n]) .. KVM_REG_ARM_CORE(fp_regs.v[n])+3.
nit: KVM_REG_ARM_CORE_REG doesn't specify the size, so I think this would be more clearly stated as "aliases KVM_REG_SIZE_U128 | KVM_REG_ARM_CORE(fp_regs.vregs[n])" or as "aliases kvm_regs.regs.fp_regs.vregs[n]".
I thought the size was effectively hard-coded to be 64 bits for KVM_REG_ARM_CORE(). Or did I misunderstand?
It's not. KVM_REG_ARM_CORE only encodes the 'group' of registers. Userspace then has to OR on the size. So the 128 bits FP regs are accessed with the following logic in QEMU:
#define AARCH64_SIMD_CORE_REG(x) \ (KVM_REG_ARM64 | KVM_REG_SIZE_U128 | \ KVM_REG_ARM_CORE | KVM_REG_ARM_CORE_REG(x))
reg.id = AARCH64_SIMD_CORE_REG(fp_regs.vregs[i]);
Looking back, the macros in kvm.h are potentially a bit silly, and should rather have a full AARCH64_SIMD_CORE_REG() definition (maybe we should add that), but the idea is to use the offset into the kvm structure as an indication for *which* register to retrieve, and then encode the size of the register at the same time, and provide a pointer to a value of the same size, which should avoid any endianness problems etc.
After another look at arch/arm64/kvm/guest.c:get_core_reg(), I get this now.
It's simplest for userspace if the two views always appear to be in sync, but it's unclear whether this is really useful. Perhaps this can be relaxed if it's a big deal for the KVM implementation; I don't know yet.
It's not going to be a big deal, and it's the only sensible thing to do; otherwise we'll have no clear semantics of where the values are. If KVM chooses to duplicate the 128 bits of state in memory, then it must also know how to syncrhonize that duplicated state when running the guest, and therefore can also do this in software on SET/GET.
I tried to remember my full rationale and describe what the rules would be if the two views are allowed to be incoherent ... and concluded that you are probably correct. This also matches the architectural view of the registers, which is what users would naturally expect.
Some early experiences taught us that anything we expose to user space should really be as close to the architectural concepts as possible, otherwise we quickly venture into land of ambiguity, which is a terrible place to be for a kernel<->userspace ABI.
Agreed!
Software intentionally modifying the registers would need to know about this aliasing, but since software is explicitly required to enable the KVM_ARM_VCPU_SVE feature, this shouldn't pose problems for backwards compatibility.
Agreed.
Vector length control:
[...]
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, __u64 vqs[SVE_VQ_MAX / 64]);
I don't understand the proposed semantics here. What is the vqs array and how should the values be set?
vqs is a bitmap, where bit (n - 1) indicates support for VL = 128n bits.
Since the hardware is not required to support all possible vector lengths up to the maximum, a bitmap seemed the most straightforward way of representing the possibly sparse set of supported VLs.
Calling it vq_bitmap[] would make this clearer.
Either way, I just think the details need to be spelled out.
Sure. This won't be the final documentation, but I'll bear it in mind. The vq bitmap concept is the same as used to track the supported vector lengths in fpsimd.c, but sane people are probably not aware of that.
Describing only the maximum supported vl turns out to be insufficient.
[...]
At present, other than initialising each vcpu to the maximum supportable set of VLs, I don't propose having a way to probe for what sets of VLs are supportable: the above call either succeeds or fails.
I think we should have such a way. Libvirt can do things like figure out compatible features across nodes, and trying to set all possible vector lengths doesn't seem like a great approach.
How about creating the interface based on a bitmap of all the possible lengths, and setting the bits for all lengths, and reading back the value returns the actual supported lengths?
Would it be sufficient for an initial call to KVM_ARM_VCPU_GET_SVE_VLS to return the default (i.e., maximum supported) set?
This would mean that once KVM_ARM_VCPU_SET_SVE_VLS has been called to set a different set, it would no longer be possible to find out the maximum set. I'm not sure this would matter, though.
I don't particularly like this, because the init sequence in QEMU can be pretty complicated, and I think it can become pretty nasty to have to remember if we 'once upon a time' called some ioctl, because then calling it now will mean something else.
In case my suggestion wasn't clear, this is what I meant:
#define NR_VQS ((32 * 2048 / 128) / 64)
__u64 vqs[NR_VQS] = { [0 ... NR_VQS - 1] = ~0 };
ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs); ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
for (i = 0; i < 16; i++) if (vqs[0] & BIT(i)) printf("vector length %d supported by KVM\n", (i+1) * 128);
This does mean that if the caller doesn't get the set they asked for in KVM_ARM_SVE_SET_VLS this is not treated as an error by the kernel. I was concerned this would encourage userspace to silently miss this error.
Good point.
What if KVM_ARM_SVE_SET_VLS() were to yield 0 if the exact requested set of VLs was configured, -ERANGE if some subset was configured successfully (but not exactly the set requested), and the usual -EINVAL/-EIO/whatever if the set of VLs couldn't be configured at all?
Sounds good to me.
Then the probe would go like this:
__u64 vqs[SVE_VQ_MAX / 64] = { [0 ... SVE_VQ_MAX / 64 - 1] = ~(u64)0 }; if (ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs) && errno != ERANGE)) goto error;
ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
/* ... */
Another option would be for SVE_ARM_SVE_SET_VLS to write the resulting set back to its argument, possibly with the same 0/ERANGE/EINVAL semantics.
Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS, and check the resulting set:
/* ... */
__u64 newvqs[SVE_VQ_MAX / 64]; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs);
if (memcmp(vqs, newvqs, sizeof vqs)) goto mismatch;
vcpu restore would need to treat any mismatch as an error: the exact requested set but be configurable, or the VM may go wrong.
I'm not sure I can parse this sentence or extract the meaning?
Any opinion on which approach is best?
I think I prefer letting KVM_ARM_SVE_SET_VLS change the supplied vqs, since having it be two separate ioctls always potentially leaves room for some other thread having modified the set in the meantime (or making a programmer doubt if this can be the case) where a single ioctl() will look atomic. The user can always call KVM_ARM_SVE_GET_VLS afterwards and should get the same result. I think it's worth trying to write this up as patches to the KVM documentation and to the kernel and see what people say on that.
I was fighting with this in December, trying to come up with a way for userspace to specify which VLs it requires/permits/forbids and letting the kernel come up with something matching these constraints. But this seemed too expressive and complex, so I had dropped the idea in favour of something simpler.
Thanks, -Christoffer
On Wed, Feb 07, 2018 at 03:58:31PM +0100, Christoffer Dall wrote:
On Wed, Feb 07, 2018 at 11:33:28AM +0000, Dave Martin wrote:
[...]
What if KVM_ARM_SVE_SET_VLS() were to yield 0 if the exact requested set of VLs was configured, -ERANGE if some subset was configured successfully (but not exactly the set requested), and the usual -EINVAL/-EIO/whatever if the set of VLs couldn't be configured at all?
Sounds good to me.
Then the probe would go like this:
__u64 vqs[SVE_VQ_MAX / 64] = { [0 ... SVE_VQ_MAX / 64 - 1] = ~(u64)0 }; if (ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs) && errno != ERANGE)) goto error;
ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
/* ... */
Another option would be for SVE_ARM_SVE_SET_VLS to write the resulting set back to its argument, possibly with the same 0/ERANGE/EINVAL semantics.
Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS, and check the resulting set:
/* ... */
__u64 newvqs[SVE_VQ_MAX / 64]; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs);
if (memcmp(vqs, newvqs, sizeof vqs)) goto mismatch;
vcpu restore would need to treat any mismatch as an error: the exact requested set but be configurable, or the VM may go wrong.
I'm not sure I can parse this sentence or extract the meaning?
That was lazy language on my part. I'll try to explain it better: When the saved state of a migrating vcpu is being loaded into a newly-created vcpu on the target node, userspace needs a way to ensure that the set of VLs that vcpu will see when it runs is _exactly_ the same set it could see before migration. I called this out separately because it's different from the case of creating a brand-new VM: in the latter case, we can't the kernel to provide the best set of VLs possible, but it is not an error not to get every VL we asked for.
Any opinion on which approach is best?
I think I prefer letting KVM_ARM_SVE_SET_VLS change the supplied vqs, since having it be two separate ioctls always potentially leaves room for some other thread having modified the set in the meantime (or making a programmer doubt if this can be the case) where a single ioctl() will look atomic.
The user can always call KVM_ARM_SVE_GET_VLS afterwards and should get the same result.
OK, I'll go with changing the supplied vqs for KVM_ARM_SVE_SET_VLS, but I'll retain the -ERANGE semantics (even if technically redundant) since that's harder to forget to check.
I think it's worth trying to write this up as patches to the KVM documentation and to the kernel and see what people say on that.
OK. Thanks for the input. Cheers ---Dave
On Wed, Feb 07, 2018 at 03:34:03PM +0000, Dave Martin wrote:
On Wed, Feb 07, 2018 at 03:58:31PM +0100, Christoffer Dall wrote:
On Wed, Feb 07, 2018 at 11:33:28AM +0000, Dave Martin wrote:
[...]
What if KVM_ARM_SVE_SET_VLS() were to yield 0 if the exact requested set of VLs was configured, -ERANGE if some subset was configured successfully (but not exactly the set requested), and the usual -EINVAL/-EIO/whatever if the set of VLs couldn't be configured at all?
Sounds good to me.
Then the probe would go like this:
__u64 vqs[SVE_VQ_MAX / 64] = { [0 ... SVE_VQ_MAX / 64 - 1] = ~(u64)0 }; if (ioctl(vcpu_fd, KVM_ARM_SVE_SET_VLS, vqs) && errno != ERANGE)) goto error;
ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, vqs);
/* ... */
Another option would be for SVE_ARM_SVE_SET_VLS to write the resulting set back to its argument, possibly with the same 0/ERANGE/EINVAL semantics.
Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS, and check the resulting set:
/* ... */
__u64 newvqs[SVE_VQ_MAX / 64]; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs);
if (memcmp(vqs, newvqs, sizeof vqs)) goto mismatch;
vcpu restore would need to treat any mismatch as an error: the exact requested set but be configurable, or the VM may go wrong.
I'm not sure I can parse this sentence or extract the meaning?
That was lazy language on my part. I'll try to explain it better:
When the saved state of a migrating vcpu is being loaded into a newly-created vcpu on the target node, userspace needs a way to ensure that the set of VLs that vcpu will see when it runs is _exactly_ the same set it could see before migration.
Yes, and that should work fine with the -ERANGE proposal, right?
I called this out separately because it's different from the case of creating a brand-new VM: in the latter case, we can't the kernel to provide the best set of VLs possible, but it is not an error not to get every VL we asked for.
This was another hard one:) I think what you're saying is that it's technically not an error when setting the VLs on a new VM, but it would be in the case of migration, and therefore we need to tell userspace in both cases what happened, and it can decide.
Any opinion on which approach is best?
I think I prefer letting KVM_ARM_SVE_SET_VLS change the supplied vqs, since having it be two separate ioctls always potentially leaves room for some other thread having modified the set in the meantime (or making a programmer doubt if this can be the case) where a single ioctl() will look atomic.
The user can always call KVM_ARM_SVE_GET_VLS afterwards and should get the same result.
OK, I'll go with changing the supplied vqs for KVM_ARM_SVE_SET_VLS, but I'll retain the -ERANGE semantics (even if technically redundant) since that's harder to forget to check.
Agree, that should definitely be part of it. Thanks, -Christoffer
On Wed, Feb 07, 2018 at 05:17:50PM +0100, Christoffer Dall wrote:
On Wed, Feb 07, 2018 at 03:34:03PM +0000, Dave Martin wrote:
On Wed, Feb 07, 2018 at 03:58:31PM +0100, Christoffer Dall wrote:
On Wed, Feb 07, 2018 at 11:33:28AM +0000, Dave Martin wrote:
[...]
Alternatively, userspace would be require to do a KVM_ARM_SVE_GET_VLS, and check the resulting set:
/* ... */
__u64 newvqs[SVE_VQ_MAX / 64]; ioctl(vcpu_fd, KVM_ARM_SVE_GET_VLS, newvqs);
if (memcmp(vqs, newvqs, sizeof vqs)) goto mismatch;
vcpu restore would need to treat any mismatch as an error: the exact requested set but be configurable, or the VM may go wrong.
I'm not sure I can parse this sentence or extract the meaning?
That was lazy language on my part. I'll try to explain it better:
When the saved state of a migrating vcpu is being loaded into a newly-created vcpu on the target node, userspace needs a way to ensure that the set of VLs that vcpu will see when it runs is _exactly_ the same set it could see before migration.
Yes, and that should work fine with the -ERANGE proposal, right?
Yes, that's the idea.
I called this out separately because it's different from the case of creating a brand-new VM: in the latter case, we can't the kernel to provide the best set of VLs possible, but it is not an error not to get every VL we asked for.
This was another hard one:)
I think what you're saying is that it's technically not an error when setting the VLs on a new VM, but it would be in the case of migration, and therefore we need to tell userspace in both cases what happened, and it can decide.
Any opinion on which approach is best?
I think I prefer letting KVM_ARM_SVE_SET_VLS change the supplied vqs, since having it be two separate ioctls always potentially leaves room for some other thread having modified the set in the meantime (or making a programmer doubt if this can be the case) where a single ioctl() will look atomic.
The user can always call KVM_ARM_SVE_GET_VLS afterwards and should get the same result.
OK, I'll go with changing the supplied vqs for KVM_ARM_SVE_SET_VLS, but I'll retain the -ERANGE semantics (even if technically redundant) since that's harder to forget to check.
Agree, that should definitely be part of it.
Agreed. Cheers ---Dave
participants (2)
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Christoffer Dall -
Dave Martin