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

[Eric Auger]

Hi Kashyap,
On 10/25/24 16:51, Kashyap Chamarthy wrote:
> 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".)
agreed, as replied earlier, custom terminology most probably will be
dropped.
>
> (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."
indeed
>
>     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 :-)

Yes named models implementing a baseline are the end goal.
>
> [...]
>
>> - 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 :-)
yes this is definitively because on ARM there is no such named KVM model
besides cortex-a57 on aarch64.
on x86 does it return the closest named model?

Thanks

Eric
>
> [...]
>
> [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
>
>