Re: [PATCH 0/7] i386: Add `machine` parameter to query-cpu-definitions

[Eduardo Habkost]

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