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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, 9 Jul 2024 08:40:18 +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, 09 Jul 2024 06:40:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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?
> 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
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