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 15:57
> 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 16:42, <Paul.Durrant@xxxxxxxxxx> wrote:
> >>  -----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?
> 
> Not that I know of. It just seems odd not to populate bits we can
> populate, simply _assuming_ that the Windows guys would not have
> added them without reason (i.e. _they_ know they can do whatever
> it is better with that knowledge).
> 

Maybe. The spec. certainly implies that no Xen guest is ever likely to do 
anything useful with them though. I'm also concerned about these bits suddenly 
becoming visible across a migrate.

  Paul

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