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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> 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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