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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Fri, Aug 1, 2014 at 10:57 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> 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).

Does Hyper-V actually expose these bits to non-root partitions (i.e.,
the viridian equivalent of domUs)?

 -George

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel