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

["Alejandro Vallejo" <alejandro.vallejo@cloud.com>]

I'll pitch in, seeing as I created the GitLab ticket.

On Tue Jul 9, 2024 at 7:40 AM BST, 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?

That would create an avoidable dependency between OVMF and xenstore, precluding
xenstoreless UEFI-enabled domUs.

>
> > This patch exposes the max_vcpus field via cpuid on the HVM hypervisor
> > leaf in edx.
>
> If exposing via CPUID, why only for HVM?
>
> > --- 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.
>
> Jan

Using max_vcpu_id instead of max_vcpus is also fine, but the flag is important
as otherwise it's impossible to retroactively change the meaning of EDX (i.e: to
stop advertising this datum, or repurpose EDX altogether)

We could also reserve only the lower 16bits of EDX rather than the whole thing;
but we have plenty of subleafs for growth, so I'm not sure it's worth it.

Cheers,
Alejandro