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Re: [RFC XEN PATCH v2] x86/cpuid: Expose max_vcpus field in HVM hypervisor leaf
- To: Matthew Barnes <matthew.barnes@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 24 Jul 2024 07:42:19 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 24 Jul 2024 05:42:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
(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)?
Jan
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