Re: [RFC 00/21] kvm/arm: Introduce a customizable aarch64 KVM host model

[Kashyap Chamarthy <kchamart@redhat.com>]

On Fri, Oct 25, 2024 at 12:17:19PM +0200, Eric Auger wrote:

Hi Eric,

I'm new to Arm, so please bear with my questions :)

> This RFC series introduces a KVM host "custom" model.

(a) On terminology: as we know, in the x86 world, QEMU uses these
    terms[1]:

    - Host passthrough
    - Named CPU models
    - Then there's the libvirt abstraction, "host-model", that aims to
      provide the best of 'host-passthrough' + named CPU models.

    Now I see the term "host 'custom' model" here.  Most
    management-layer tools and libvirt users are familiar with the
    classic terms "host-model" or "custom".  If we now say "host
    'custom' model", it can create confusion.  I hope we can settle on
    one of the existing terms, or create a new term if need be.

    (I'll share one more thought on how layers above libvirt tend to use
    the term "custom", as a reply to patch 21/21, "arm/cpu-features:
    Document custom vcpu model".)

(b) The current CPU features doc[2] for Arm doesn't mention "host
    passthrough" at all.  It is only implied by the last part of this
    paragraph, from the section titled "A note about CPU models and
    KVM"[3]:

      "Named CPU models generally do not work with KVM. There are a few
      cases that do work [...] but mostly if KVM is enabled the 'host'
      CPU type must be used."

    Related: in your reply[4] to Dan in this series, you write: "Having
    named models is the next thing".  So named CPU models will be a
    thing in Arm, too?  Then the above statement in the Arm
    'cpu-features' will need updating :-)

[...]

> - the QEMU layer does not take care of IDREG field value consistency.
>   The kernel neither. I imagine this could be the role of the upper
>   layer to implement a vcpu profile that makes sure settings are
>   consistent. Here we come to "named" models. What should they look
>   like on ARM?

Are there reasons why they can't be similar to how x86 reports in
`qemu-system-x86 -cpu help`?  

E.g. If it's an NVIDIA "Grace A02" (Neoverse-V2) host, it can report:

    [gracehopper] $> qemu-kvm -cpu help
    Available CPUs:
      gracehopper-neoverse-v2
      cortex-a57 (deprecated)
      host
      max

Or whatever is the preferred nomenclature for ARM.  It also gives users
of both x86 and ARM deployments a consistent expectation.  

Currently on a "Grace A02" ("Neoverse-V2") machine, it reports:

    [gracehopper] $> qemu-kvm -cpu help
    Available CPUs:
      cortex-a57 (deprecated)
      host
      max

I see it's because there are no named models yet on ARM :-)

[...]

[1] https://www.qemu.org/docs/master/system/i386/cpu.html
[2] https://www.qemu.org/docs/master/system/arm/cpu-features.html
[3] https://www.qemu.org/docs/master/system/arm/cpu-features.html#a-note-about-cpu-models-and-kvm
[4] https://lists.nongnu.org/archive/html/qemu-arm/2024-10/msg00891.html


-- 
/kashyap