Re: [Xen-devel] [PATCH v2] x86/hvm: Extend HVM cpuid leaf with vcpu id

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Matt Wilson [mailto:mswilson@xxxxxxxxx] On Behalf Of Matt Wilson
> Sent: 11 December 2014 23:17
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; Keir (Xen.org); Jan Beulich
> Subject: Re: [Xen-devel] [PATCH v2] x86/hvm: Extend HVM cpuid leaf with
> vcpu id
> 
> On Thu, Nov 06, 2014 at 03:26:24PM +0000, Paul Durrant wrote:
> > To perform certain hypercalls HVM guests need to use Xen's idea of
> > vcpu id, which may well not match the guest OS idea of CPU id.
> > This patch adds vcpu id to the HVM cpuid leaf allowing the guest
> > to build a mapping.
> 
> One more tangent on this, since this is partially considered in a
> Windows context. Why not use the existing Viridian virtual processor
> index MSR for this, since that would work on Xen today.
> 
> xen/arch/x86/hvm/viridian.c:
> > #define VIRIDIAN_MSR_VP_INDEX                   0x40000002
> > ...
> >     case VIRIDIAN_MSR_VP_INDEX:
> >        perfc_incr(mshv_rdmsr_vp_index);
> >        *val = v->vcpu_id;
> >        break;
> 

Yes, that would work for VMs where viridian is enabled, but there's no 
requirement that viridian is enabled to install PV drivers even in Windows. I 
guess ACPI is the only robust way to get vcpu id in an HVM guest in the absence 
of the new cpuid leaf.

  Paul

> --msw

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