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

[Matthew Barnes <matthew.barnes@cloud.com>]

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?

> > --- a/xen/include/public/arch-x86/cpuid.h
> > +++ b/xen/include/public/arch-x86/cpuid.h
> > @@ -87,6 +87,7 @@
> >   * Sub-leaf 0: EAX: Features
> >   * Sub-leaf 0: EBX: vcpu id (iff EAX has XEN_HVM_CPUID_VCPU_ID_PRESENT flag)
> >   * Sub-leaf 0: ECX: domain id (iff EAX has XEN_HVM_CPUID_DOMID_PRESENT flag)
> > + * Sub-leaf 0: EDX: max vcpus (iff EAX has XEN_HVM_CPUID_MAX_VCPUS_PRESENT flag)
> >   */
> 
> Unlike EBX and ECX, the proposed value for EDX cannot be zero. I'm therefore
> not entirely convinced that we need a qualifying flag. Things would be
> different if the field was "highest possible vCPU ID", which certainly would
> be the better approach if the field wasn't occupying the entire register.
> Even with it being 32 bits, I'd still suggest switching its meaning this way.

I shall tweak the value from "maximum vcpu count" to "maximum vcpu ID"
in patch v2.

Matt