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Re: [RFC XEN PATCH] x86/cpuid: Expose max_vcpus field in HVM hypervisor leaf
- To: Matthew Barnes <matthew.barnes@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 16 Jul 2024 17:37:10 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 16 Jul 2024 15:37:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 16.07.2024 17:03, Matthew Barnes wrote:
> On Tue, Jul 09, 2024 at 08:40:18AM +0200, Jan Beulich wrote:
>> On 08.07.2024 17:42, 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.
>>>
>>> For OVMF to access the maximum vCPU count, Xen will have to expose this
>>> property via cpuid.
>>
>> Why "have to"? The information is available from xenstore, isn't it?
>
> I shall reword the commit message in patch v2 to avoid the wording "have
> to".
>
>>> This patch exposes the max_vcpus field via cpuid on the HVM hypervisor
>>> leaf in edx.
>>
>> If exposing via CPUID, why only for HVM?
>
> Other related cpuid fields are also exposed in the HVM hypervisor leaf,
> such as the vcpu id and the domain id.
>
> Having said that, I wouldn't mind moving this field (or other fields, in
> a separate patch) to a location meant for HVM *and* PV guests. Do you
> have any suggestions?
I don't think we can literally move anything. We could duplicate things
into shared fields, but would that gain us anything? Therefore I think
going forward we should limit type-specific fields to cases where type
really matters, and otherwise expose in a type-independent way.
Jan
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