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
