10:08 a.m.
CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
that the definition of "supported CPU features" will probably become a
bit more complex in the future.
On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini <pbonzini(a)redhat.com> wrote:
On 06/07/21 23:33, Eduardo Habkost wrote:
> On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini(a)redhat.com> wrote:
>> It's a bit tricky, because HLE and RTM won't really behave well. An
old
>> guest that sees RTM=1 might end up retrying and aborting transactions
>> too much. So I'm not sure that a QEMU "-cpu host" guest should
have HLE
>> and RTM enabled.
>
> Is the purpose of GET_SUPPORTED_CPUID to return what is supported by
> KVM, or to return what "-cpu host" should enable by default? They are
> conflicting requirements in this case.
In theory there is GET_EMULATED_CPUID for the former, so it should be
the latter. In practice neither QEMU nor Libvirt use it; maybe now we
have a good reason to add it, but note that userspace could also check
host RTM_ALWAYS_ABORT.
> Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace
> take bad decisions until it's updated.
>
> Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing
> userspace from resuming existing VMs (despite being technically
> possible).
>
> The first option has an easy workaround that doesn't require a
> software update (disabling HLE/RTM in the VM configuration). The
> second option doesn't have a workaround. I'm inclined towards the
> first option.
The default has already been tsx=off for a while though, so checking
either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might
also be feasible for those that are still on tsx=on.
This sounds like a perfect use case for GET_EMULATED_CPUID. My only
concern is breaking existing userspace.
But if this was already broken for a few kernel releases due to
tsx=off being the default, maybe GET_EMULATED_CPUID will be a
reasonable approach.
--
Eduardo
11:42 a.m.
New subject: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
that the definition of "supported CPU features" will probably become a
bit more complex in the future.
Has there ever been a clear definition? Family, model, and stepping,
for instance: are these the only values supported? That would make
cross-platform migration impossible. What about the vendor string? Is
that the only value supported? That would make cross-vendor migration
impossible. For the maximum input value for basic CPUID information
(CPUID.0H:EAX), is that the only value supported, or is it the maximum
value supported? On the various individual feature bits, does a '1'
imply that '0' is also supported, or is '1' the only value supported?
What about the feature bits with reversed polarity (e.g.
CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
This API has never made sense to me. I have no idea how to interpret
what it is telling me.
On Tue, Jul 6, 2021 at 5:58 PM Paolo Bonzini
<pbonzini(a)redhat.com> wrote:
>
> On 06/07/21 23:33, Eduardo Habkost wrote:
> > On Tue, Jul 6, 2021 at 5:05 PM Paolo Bonzini <pbonzini(a)redhat.com>
wrote:
> >> It's a bit tricky, because HLE and RTM won't really behave well.
An old
> >> guest that sees RTM=1 might end up retrying and aborting transactions
> >> too much. So I'm not sure that a QEMU "-cpu host" guest
should have HLE
> >> and RTM enabled.
> >
> > Is the purpose of GET_SUPPORTED_CPUID to return what is supported by
> > KVM, or to return what "-cpu host" should enable by default? They
are
> > conflicting requirements in this case.
>
> In theory there is GET_EMULATED_CPUID for the former, so it should be
> the latter. In practice neither QEMU nor Libvirt use it; maybe now we
> have a good reason to add it, but note that userspace could also check
> host RTM_ALWAYS_ABORT.
>
> > Returning HLE=1,RTM=1 in GET_SUPPORTED_CPUID makes existing userspace
> > take bad decisions until it's updated.
> >
> > Returning HLE=0,RTM=0 in GET_SUPPORTED_CPUID prevents existing
> > userspace from resuming existing VMs (despite being technically
> > possible).
> >
> > The first option has an easy workaround that doesn't require a
> > software update (disabling HLE/RTM in the VM configuration). The
> > second option doesn't have a workaround. I'm inclined towards the
> > first option.
>
> The default has already been tsx=off for a while though, so checking
> either GET_EMULATED_CPUID or host RTM_ALWAYS_ABORT in userspace might
> also be feasible for those that are still on tsx=on.
This sounds like a perfect use case for GET_EMULATED_CPUID. My only
concern is breaking existing userspace.
But if this was already broken for a few kernel releases due to
tsx=off being the default, maybe GET_EMULATED_CPUID will be a
reasonable approach.
--
Eduardo
12:08 p.m.
New subject: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On Wed, Jul 7, 2021 at 12:42 PM Jim Mattson <jmattson(a)google.com> wrote:
On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
>
> CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
> that the definition of "supported CPU features" will probably become a
> bit more complex in the future.
Has there ever been a clear definition? Family, model, and stepping,
for instance: are these the only values supported? That would make
cross-platform migration impossible. What about the vendor string? Is
that the only value supported? That would make cross-vendor migration
impossible. For the maximum input value for basic CPUID information
(CPUID.0H:EAX), is that the only value supported, or is it the maximum
value supported? On the various individual feature bits, does a '1'
imply that '0' is also supported, or is '1' the only value supported?
What about the feature bits with reversed polarity (e.g.
CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
This API has never made sense to me. I have no idea how to interpret
what it is telling me.
Is this about GET_SUPPORTED_CPUID, QEMU's query-cpu-model-expansion &
related commands, or the libvirt CPU APIs?
--
Eduardo
12:15 p.m.
New subject: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On Wed, Jul 7, 2021 at 10:08 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
On Wed, Jul 7, 2021 at 12:42 PM Jim Mattson <jmattson(a)google.com> wrote:
>
> On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
> >
> > CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
> > that the definition of "supported CPU features" will probably become
a
> > bit more complex in the future.
>
> Has there ever been a clear definition? Family, model, and stepping,
> for instance: are these the only values supported? That would make
> cross-platform migration impossible. What about the vendor string? Is
> that the only value supported? That would make cross-vendor migration
> impossible. For the maximum input value for basic CPUID information
> (CPUID.0H:EAX), is that the only value supported, or is it the maximum
> value supported? On the various individual feature bits, does a '1'
> imply that '0' is also supported, or is '1' the only value
supported?
> What about the feature bits with reversed polarity (e.g.
> CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
>
> This API has never made sense to me. I have no idea how to interpret
> what it is telling me.
Is this about GET_SUPPORTED_CPUID, QEMU's query-cpu-model-expansion &
related commands, or the libvirt CPU APIs?
This is my ongoing rant about KVM_GET_SUPPORTED_CPUID.
1:23 p.m.
New subject: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On Wed, Jul 7, 2021 at 1:18 PM Jim Mattson <jmattson(a)google.com> wrote:
On Wed, Jul 7, 2021 at 10:08 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
>
> On Wed, Jul 7, 2021 at 12:42 PM Jim Mattson <jmattson(a)google.com> wrote:
> >
> > On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com>
wrote:
> > >
> > > CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
> > > that the definition of "supported CPU features" will probably
become a
> > > bit more complex in the future.
> >
> > Has there ever been a clear definition? Family, model, and stepping,
> > for instance: are these the only values supported? That would make
> > cross-platform migration impossible. What about the vendor string? Is
> > that the only value supported? That would make cross-vendor migration
> > impossible. For the maximum input value for basic CPUID information
> > (CPUID.0H:EAX), is that the only value supported, or is it the maximum
> > value supported? On the various individual feature bits, does a '1'
> > imply that '0' is also supported, or is '1' the only value
supported?
> > What about the feature bits with reversed polarity (e.g.
> > CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
> >
> > This API has never made sense to me. I have no idea how to interpret
> > what it is telling me.
>
> Is this about GET_SUPPORTED_CPUID, QEMU's query-cpu-model-expansion &
> related commands, or the libvirt CPU APIs?
This is my ongoing rant about KVM_GET_SUPPORTED_CPUID.
I agree the definition is not clear. I have tried to enumerate below
what QEMU assumes about the return value of KVM_GET_SUPPORTED_CPUID.
These are a collection of workarounds and feature-specific rules that
are encoded in the kvm_arch_get_supported_cpuid()
x86_cpu_filter_features(), and cpu_x86_cpuid() functions in QEMU.
1. Passing through the returned values (unchanged) from
KVM_GET_SUPPORTED_CPUID to KVM_SET_CPUID is assumed to be always safe,
as long as the ability to save/resume VCPU state is not required.
(This is the behavior implemented by "-cpu host,migratable=off")
2. The safety of setting a bit to a different value requires specific
knowledge about the CPUID bit.
2.1. For a specific set of registers (see below), QEMU assumes it's
safe to set the bit to 0 when KVM_GET_SUPPORTED_CPUID returns 1.
2.2. For a few specific leaves (see below), there are more complex rules.
2.4. For all other leaves, QEMU doesn't use the return value of
KVM_GET_SUPPORTED_CPUID at all (AFAICS).
The CPUID leaves mentioned in 2.1 are:
CPUID[1].EDX
CPUID[1].ECX
CPUID[6].EAX
CPUID[EAX=7,ECX=0].EBX
- This unfortunately includes de-feature bits like FDP_EXCPTN_ONLY and
ZERO_FCS_FDS
CPUID[EAX=7,ECX=0].ECX
CPUID[EAX=7,ECX=0].EDX
CPUID[EAX=7,ECX=1].EAX
CPUID[EAX=0Dh,ECX=0].EAX
CPUID[EAX=0Dh,ECX=0].EDX
CPUID[EAX=0Dh,ECX=1].EAX
- Note that CPUID[0Dh] has additional logic to ensure XSAVE component
info on CPUID is consistent
CPUID[40000001h].EAX
CPUID[40000001h].EDX
CPUID[80000001h].EDX
CPUID[80000001h].ECX
CPUID[80000007h].EDX
CPUID[80000008h].EBX
CPUID[8000000Ah].EDX
CPUID[C0000001h].EDX
Some of the CPUID leaves mentioned in 2.2 are:
CPUID[1].ECX.HYPERVISOR[bit 31]
- Can be enabled unconditionally
CPUID[1].ECX.TSC_DEADLINE_TIMER[bit 24]
- Can be set to 1 if using the in-kernel irqchip and
KVM_CAP_TSC_DEADLINE_TIMER is enabled
CPUID[1].ECX.X2APIC[bit 21]
- Can be set to 1 if using the in-kernel irqchip
CPUID[1].ECX.MONITOR[bit 3]
- Can be set to 1 if KVM_X86_DISABLE_EXITS_MWAIT is enabled
CPUID[6].EAX.ARAT[bit 2]
- Can be enabled unconditionally
CPUID[EAX=7,ECX=0].EDX.ARCH_CAPABILITIES
- Workaround for KVM bug in Linux v4.17-v4.20
CPUID[EAX=14h,ECX=0], CPUID{EAX=14h,ECX=1]
- Most bits must match the host, unless
CPUID[EAX=7,ECX=0].EBX.INTEL_PT[bit 25] is 0
CPUID[80000001h].EDX
- AMD-specific feature flag aliases can be set based on CPUID[1].EDX
CPUID[40000001h].EAX
- KVM_FEATURE_PV_UNHALT requires in-kernel irqchip
- KVM_FEATURE_MSI_EXT_DEST_ID requires split irqchip
CPUID[40000001].EDX.KVM_HINTS_REALTIME
- Can be enabled unconditionally
--
Eduardo
4:15 a.m.
New subject: [PATCH 4/4] x86/tsx: Add cmdline tsx=fake to not clear CPUID bits RTM and HLE
On 07/07/21 20:23, Eduardo Habkost wrote:
On Wed, Jul 7, 2021 at 1:18 PM Jim Mattson
<jmattson(a)google.com> wrote:
>
> On Wed, Jul 7, 2021 at 10:08 AM Eduardo Habkost <ehabkost(a)redhat.com> wrote:
>>
>> On Wed, Jul 7, 2021 at 12:42 PM Jim Mattson <jmattson(a)google.com> wrote:
>>>
>>> On Wed, Jul 7, 2021 at 8:09 AM Eduardo Habkost <ehabkost(a)redhat.com>
wrote:
>>>>
>>>> CCing libvir-list, Jiri Denemark, Michal Privoznik, so they are aware
>>>> that the definition of "supported CPU features" will probably
become a
>>>> bit more complex in the future.
>>>
>>> Has there ever been a clear definition? Family, model, and stepping,
>>> for instance: are these the only values supported? That would make
>>> cross-platform migration impossible. What about the vendor string? Is
>>> that the only value supported? That would make cross-vendor migration
>>> impossible. For the maximum input value for basic CPUID information
>>> (CPUID.0H:EAX), is that the only value supported, or is it the maximum
>>> value supported? On the various individual feature bits, does a '1'
>>> imply that '0' is also supported, or is '1' the only value
supported?
>>> What about the feature bits with reversed polarity (e.g.
>>> CPUID.(EAX=07H,ECX=0):EBX.FDP_EXCPTN_ONLY[bit 6])?
>>>
>>> This API has never made sense to me. I have no idea how to interpret
>>> what it is telling me.
>>
>> Is this about GET_SUPPORTED_CPUID, QEMU's query-cpu-model-expansion &
>> related commands, or the libvirt CPU APIs?
>
> This is my ongoing rant about KVM_GET_SUPPORTED_CPUID.
>
I agree the definition is not clear. I have tried to enumerate below
what QEMU assumes about the return value of KVM_GET_SUPPORTED_CPUID.
These are a collection of workarounds and feature-specific rules that
are encoded in the kvm_arch_get_supported_cpuid()
x86_cpu_filter_features(), and cpu_x86_cpuid() functions in QEMU.
1. Passing through the returned values (unchanged) from
KVM_GET_SUPPORTED_CPUID to KVM_SET_CPUID is assumed to be always safe,
as long as the ability to save/resume VCPU state is not required.
(This is the behavior implemented by "-cpu host,migratable=off")
Right, this is basically the definition of KVM_GET_SUPPORTED_CPUID.
2. The safety of setting a bit to a different value requires
specific
knowledge about the CPUID bit.
2.1. For a specific set of registers (see below), QEMU assumes it's
safe to set the bit to 0 when KVM_GET_SUPPORTED_CPUID returns 1.
2.2. For a few specific leaves (see below), there are more complex rules.
2.4. For all other leaves, QEMU doesn't use the return value of
KVM_GET_SUPPORTED_CPUID at all (AFAICS).
The CPUID leaves mentioned in 2.1 are:
CPUID[1].EDX
CPUID[1].ECX
CPUID[6].EAX
CPUID[EAX=7,ECX=0].EBX
- This unfortunately includes de-feature bits like FDP_EXCPTN_ONLY and
ZERO_FCS_FDS
CPUID[EAX=7,ECX=0].ECX
CPUID[EAX=7,ECX=0].EDX
CPUID[EAX=7,ECX=1].EAX
CPUID[EAX=0Dh,ECX=0].EAX
CPUID[EAX=0Dh,ECX=0].EDX
CPUID[EAX=0Dh,ECX=1].EAX
- Note that CPUID[0Dh] has additional logic to ensure XSAVE component
info on CPUID is consistent
CPUID[40000001h].EAX
CPUID[40000001h].EDX
CPUID[80000001h].EDX
CPUID[80000001h].ECX
CPUID[80000007h].EDX
CPUID[80000008h].EBX
CPUID[8000000Ah].EDX
CPUID[C0000001h].EDX
Plus all unknown leaves.
Some of the CPUID leaves mentioned in 2.2 are:
CPUID[1].ECX.HYPERVISOR[bit 31]
- Can be enabled unconditionally
CPUID[1].ECX.TSC_DEADLINE_TIMER[bit 24]
- Can be set to 1 if using the in-kernel irqchip and
KVM_CAP_TSC_DEADLINE_TIMER is enabled
CPUID[1].ECX.X2APIC[bit 21]
- Can be set to 1 if using the in-kernel irqchip
CPUID[1].ECX.MONITOR[bit 3]
- Can be set to 1 if KVM_X86_DISABLE_EXITS_MWAIT is enabled
Can always be set to 1, but only makes sense to do so if
KVM_X86_DISABLE_EXITS_MWAIT is enabled.
CPUID[6].EAX.ARAT[bit 2]
- Can be enabled unconditionally
CPUID[EAX=7,ECX=0].EDX.ARCH_CAPABILITIES
- Workaround for KVM bug in Linux v4.17-v4.20
CPUID[EAX=14h,ECX=0], CPUID{EAX=14h,ECX=1]
- Most bits must match the host, unless
CPUID[EAX=7,ECX=0].EBX.INTEL_PT[bit 25] is 0
CPUID[80000001h].EDX
- AMD-specific feature flag aliases can be set based on CPUID[1].EDX
CPUID[40000001h].EAX
- KVM_FEATURE_PV_UNHALT requires in-kernel irqchip
- KVM_FEATURE_MSI_EXT_DEST_ID requires split irqchip
CPUID[40000001].EDX.KVM_HINTS_REALTIME
- Can be enabled unconditionally
This should apply to all of CPUID[4000_0001h].EDX in the future
Thanks Eduardo, this is a great start for kernel-side documentation!
I'll wrap it in a patch.
Paolo
1233
days inactive
1234
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5 comments
3 participants
participants (3)
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Eduardo Habkost -
Jim Mattson -
Paolo Bonzini