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

[Matthew Barnes <matthew.barnes@xxxxxxxxx>]

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?

Matt