Re: [Xen-devel] [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 01 August 2014 15:16
> To: Paul Durrant
> Cc: Andrew Cooper; xen-devel; Keir (Xen.org)
> Subject: RE: [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits
> 
> >>> On 01.08.14 at 15:58, <Paul.Durrant@xxxxxxxxxx> wrote:
> >>  -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: 01 August 2014 14:49
> >> To: xen-devel
> >> Cc: Andrew Cooper; Paul Durrant; Keir (Xen.org)
> >> Subject: [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >
> > Whilst this patch is technically fine, is it of any real use? From my
> > reading of the Hypervisor Top Level Functional Spec (v4.0a) these bits are
> > only of relevance to the root partition.
> 
> I'm not that familiar with Hyper-V concepts, so I'm not sure what "root
> partition" refers to. If it was what I imagined (the host OS instance),
> then these bits clearly couldn't be meant for it, as it only sees the
> native CPUID output.
> 

I'm not sure that's true. I think Hyper-V runs its root partition (i.e. control 
domain) in a VM container and so it doesn't see native CPUID either. Anyway, 
what is the benefit of adding these two bits in Xen's viridian code? Have they 
actually been observed to make a difference to a Windows guest?

  Paul

> > If they do make a difference to a guest, should we not also be specifying
> > bits 6 and 7 in leaf 40000004:EAX
> 
> Rather not I would say, at least on VT-d, until we have a vIOMMU.
> And I'm not sure the vIOMMU on the AMD side is mature enough
> to _recommend_ its use.
> 
> > and putting a value in 40000005:ECX?
> 
> That one I'd view as separate work item, aiming at populating its
> EAX and EBX outputs at once. Albeit I may again have a terminology
> issue here: For a guest, what's the difference between virtual and
> logical CPUs? Additionally, for ECX I don't think we would know the
> correct value to put there under Xen, since we're not doing the
> remapping in a vector centric way (i.e. this value might be reflecting
> some Hyper-V internals).
> 
> Jan


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