Re: [libvirt] [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions
CCing danpb, libvir-list, mskrivanek. On Fri, Oct 25, 2019 at 09:17:46AM +0200, David Hildenbrand wrote:
On 25.10.19 04:25, Eduardo Habkost wrote:
We had introduced versioned CPU models in QEMU 4.1, including a method for querying for CPU model versions using
Interesting, I was not aware of that.
On s390x, we somewhat have versioned CPU models, but we decided against giving them explicit names (e.g., z13-v1 or z13-4.1.0), because it didn't really seem to be necessary. (and we often implement/add features for older CPU models, there is a lot of fluctuation) Actually, you would have had to add "z13-z/VM-x.x.x" or "z13-LPAR-x.x.x" or "z13-KVM-x.x.x" to model the features you actually see in all the different virtual environments ("what a CPU looks like"). Not to talk about QEMU versions in addition to that. So we decided to always model what you would see under LPAR and are able to specify for a KVM guest. So you can use "z13" in an up-to-date LPAR environment, but not in a z/VM environment (you would have to disable features).
Each (!base) CPU model has a specific feature set per machine. Libvirt uses query-cpu-model-expansion() to convert this model+machine to a machine-independent representation. That representation is sufficient for all use cases we were aware of (esp. "virsh domcapabilities", the host CPU model, migration).
While s390x has versioned CPU models, we have no explicit way of specifying them for older machines, besides going over query-cpu-model-expansion and using expanded "base model + features".
I can see that this might make sense on x86-64, where you only have maybe 3 versions of a CPU (e.g., the one Intel messed up first - Haswell, the one Intel messed up next - Haswell-noTSX, and the one that Intel eventually did right - Haswell-noTSX-IBRS) and you might want to specify "Haswell" vs. "Haswell-IBRS" vs. "Haswell-noTSX-IBRS". But actually, you will always want to go for "Haswell-noTSX-IBRS", because you can expect to run in updated environments if I am not wrong, everything else are corner cases.
Of course, versioned CPU model are neat to avoid "base model + list of features", but at least for expanding the host model on s390x, it is not really helpful. When migrating, the model expansion does the trick.
I haven't looked into details of "how to specify or model versions". Maybe IBM wants to look into creating versions for all the old models we had. But again, not sure if that is of any help for s390x. CCing IBM.
I'm not sure yet if there are the x86-specific goals and constraints that would make it difficult to follow the same approach followed by s390x. I have the impression we do, but I need to think more carefully about it. Let's discuss that during KVM Forum? The two main goals of versioned CPU models in x86 are: 1) Decoupling CPU model updates from machine-types (users should be able to update a CPU model definition without changing machine type). 2) Letting management software automate CPU model updates. Normally this is necessary when bare metal microcode or software updates add/remove features from CPUs. Sometimes the Virtual CPU model update needs to be performed before the host updates are applied (if features are being removed). The main constraint that makes it difficult to address the above without a new API is: A) Every CPU model in x86 except "host" is already expected to be migration-safe (I don't know how this compares to s390x).
query-cpu-definitions. This only has one problem: fetching CPU alias information for multiple machine types required restarting QEMU for each machine being queried.
This series adds a new `machine` parameter to query-cpu-definitions, that can be used to query CPU model alias information for multiple machines without restarting QEMU.
Eduardo Habkost (7): i386: Use g_autofree at x86_cpu_list_entry() i386: Add default_version parameter to CPU version functions i386: Don't use default_cpu_version at "-cpu help" machine: machine_find_class() function i386: Remove x86_cpu_set_default_version() function i386: Don't use default_cpu_version() inside query-cpu-definitions cpu: Add `machine` parameter to query-cpu-definitions
qapi/machine-target.json | 14 +++- include/hw/boards.h | 1 + include/hw/i386/pc.h | 5 +- target/i386/cpu.h | 6 -- hw/core/machine.c | 16 ++++ hw/i386/pc.c | 3 - target/arm/helper.c | 4 +- target/i386/cpu.c | 93 +++++++++++++++------- target/mips/helper.c | 4 +- target/ppc/translate_init.inc.c | 4 +- target/s390x/cpu_models.c | 4 +- vl.c | 17 +--- tests/acceptance/x86_cpu_model_versions.py | 42 ++++++++++ 13 files changed, 154 insertions(+), 59 deletions(-)
--
Thanks,
David / dhildenb
-- Eduardo
On 25.10.19 15:38, Eduardo Habkost wrote:
CCing danpb, libvir-list, mskrivanek.
On Fri, Oct 25, 2019 at 09:17:46AM +0200, David Hildenbrand wrote:
On 25.10.19 04:25, Eduardo Habkost wrote:
We had introduced versioned CPU models in QEMU 4.1, including a method for querying for CPU model versions using
Interesting, I was not aware of that.
On s390x, we somewhat have versioned CPU models, but we decided against giving them explicit names (e.g., z13-v1 or z13-4.1.0), because it didn't really seem to be necessary. (and we often implement/add features for older CPU models, there is a lot of fluctuation) Actually, you would have had to add "z13-z/VM-x.x.x" or "z13-LPAR-x.x.x" or "z13-KVM-x.x.x" to model the features you actually see in all the different virtual environments ("what a CPU looks like"). Not to talk about QEMU versions in addition to that. So we decided to always model what you would see under LPAR and are able to specify for a KVM guest. So you can use "z13" in an up-to-date LPAR environment, but not in a z/VM environment (you would have to disable features).
Each (!base) CPU model has a specific feature set per machine. Libvirt uses query-cpu-model-expansion() to convert this model+machine to a machine-independent representation. That representation is sufficient for all use cases we were aware of (esp. "virsh domcapabilities", the host CPU model, migration).
While s390x has versioned CPU models, we have no explicit way of specifying them for older machines, besides going over query-cpu-model-expansion and using expanded "base model + features".
I can see that this might make sense on x86-64, where you only have maybe 3 versions of a CPU (e.g., the one Intel messed up first - Haswell, the one Intel messed up next - Haswell-noTSX, and the one that Intel eventually did right - Haswell-noTSX-IBRS) and you might want to specify "Haswell" vs. "Haswell-IBRS" vs. "Haswell-noTSX-IBRS". But actually, you will always want to go for "Haswell-noTSX-IBRS", because you can expect to run in updated environments if I am not wrong, everything else are corner cases.
Of course, versioned CPU model are neat to avoid "base model + list of features", but at least for expanding the host model on s390x, it is not really helpful. When migrating, the model expansion does the trick.
I haven't looked into details of "how to specify or model versions". Maybe IBM wants to look into creating versions for all the old models we had. But again, not sure if that is of any help for s390x. CCing IBM.
I'm not sure yet if there are the x86-specific goals and constraints that would make it difficult to follow the same approach followed by s390x. I have the impression we do, but I need to think more carefully about it.
Let's discuss that during KVM Forum?
I won't be attending KVM Forum this year, but Christian should be around.
The two main goals of versioned CPU models in x86 are: 1) Decoupling CPU model updates from machine-types (users should be able to update a CPU model definition without changing machine type). 2) Letting management software automate CPU model updates. Normally this is necessary when bare metal microcode or software updates add/remove features from CPUs. Sometimes the Virtual CPU model update needs to be performed before the host updates are applied (if features are being removed)
We have 2) on s390x after a QEMU machine update. You need a QEMU update usually either way to support the new CPU feature. But I can see how decoupling the CPU model definition from the machine makes this easier. It does introduce complexity. It applies only when running older QEMU machines, or if we have to update a CPU model before we get a new QEMU machine. But we do have versioned CPU models already, so the discussion is already over :)
The main constraint that makes it difficult to address the above without a new API is: A) Every CPU model in x86 except "host" is already expected to be migration-safe (I don't know how this compares to s390x).
I would describe that not a bug but a feature :) It does come with the price that only using the newest QEMU machine results in the newest CPU model, that part I understand. -- Thanks, David / dhildenb
participants (2)
-
David Hildenbrand -
Eduardo Habkost