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

["Jan Beulich" <JBeulich@suse.com>]

>>> On 01.08.14 at 16:42, <Paul.Durrant@citrix.com> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@suse.com]
>> 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@citrix.com> wrote:
>> >>  -----Original Message-----
>> >> From: Jan Beulich [mailto:JBeulich@suse.com]
>> >> 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@suse.com>
>> >
>> > 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).

Jan