Re: [RFC XEN PATCH v2] x86/cpuid: Expose max_vcpus field in HVM hypervisor leaf

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.07.2024 14:51, Matthew Barnes wrote:
> On Wed, Jul 24, 2024 at 07:42:19AM +0200, Jan Beulich wrote:
>> (re-adding xen-devel@)
>>
>> On 23.07.2024 14:57, Matthew Barnes wrote:
>>> On Mon, Jul 22, 2024 at 01:37:11PM +0200, Jan Beulich wrote:
>>>> On 19.07.2024 16:21, Matthew Barnes wrote:
>>>>> Currently, OVMF is hard-coded to set up a maximum of 64 vCPUs on
>>>>> startup.
>>>>>
>>>>> There are efforts to support a maximum of 128 vCPUs, which would involve
>>>>> bumping the OVMF constant from 64 to 128.
>>>>>
>>>>> However, it would be more future-proof for OVMF to access the maximum
>>>>> number of vCPUs for a domain and set itself up appropriately at
>>>>> run-time.
>>>>>
>>>>> GitLab ticket: https://gitlab.com/xen-project/xen/-/issues/191
>>>>>
>>>>> For OVMF to access the maximum vCPU count, this patch has Xen expose
>>>>> the maximum vCPU ID via cpuid on the HVM hypervisor leaf in edx.
>>>>>
>>>>> Signed-off-by: Matthew Barnes <matthew.barnes@xxxxxxxxx>
>>>>> ---
>>>>> Changes in v2:
>>>>> - Tweak value from "maximum vcpu count" to "maximum vcpu id"
>>>>> - Reword commit message to avoid "have to" wording
>>>>> - Fix vpcus -> vcpus typo
>>>>> ---
>>>>
>>>> Yet still HVM-only?
>>>
>>> This field is only used when the guest is HVM, so I decided it should
>>> only be present to HVM guests.
>>>
>>> If not, where else would you suggest to put this field?
>>
>> In a presently unused leaf? Or one of the unused registers of leaf x01
>> (with the gating flag in leaf x02 ECX)?
> 
> I could establish leaf x06 as a 'domain info' leaf for both HVM and PV,
> have EAX as a features bitmap, and EBX as the max_vcpu_id field.
> 
> Is this satisfactory?

Hmm. Personally I think that all new leaves would better permit for multiple
sub-leaves. Hence EAX is already unavailable. Additionally I'm told that
there are internal discussions (supposed to be) going on at your end, which
makes me wonder whether the above is the outcome of those discussions (in
particular having at least tentative buy-off by Andrew).

For the particular data to expose here, I would prefer the indicated re-use
of an existing leaf. I haven't seen counter-arguments to that so far.

Jan