Re: [Xen-devel] Xen virtual IOMMU high level design doc V2

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.10.16 at 09:32, <tianyu.lan@xxxxxxxxx> wrote:
> On 10/21/2016 4:36 AM, Andrew Cooper wrote:
>>>>> 3.5 Implementation consideration
>>>>> VT-d spec doesn't define a capability bit for the l2 translation.
>>>>> Architecturally there is no way to tell guest that l2 translation
>>>>> capability is not available. Linux Intel IOMMU driver thinks l2
>>>>> translation is always available when VTD exits and fail to be loaded
>>>>> without l2 translation support even if interrupt remapping and l1
>>>>> translation are available. So it needs to enable l2 translation first
>>>>> before other functions.
>>>>
>>>> What then is the purpose of the nested translation support bit in the
>>>> extended capability register?
>>>
>>> It's to translate output GPA from first level translation(IOVA->GPA)
>>> to HPA.
>>>
>>> Detail please see VTD spec - 3.8 Nested Translation
>>> "When Nesting Enable (NESTE) field is 1 in extended-context-entries,
>>> requests-with-PASID translated through first-level translation are also
>>> subjected to nested second-level translation. Such extendedcontext-
>>> entries contain both the pointer to the PASID-table (which contains the
>>> pointer to the firstlevel translation structures), and the pointer to
>>> the second-level translation structures."
>>
>> I didn't phrase my question very well.  I understand what the nested
>> translation bit means, but I don't understand why we have a problem
>> signalling the presence or lack of nested translations to the guest.
>>
>> In other words, why can't we hide l2 translation from the guest by
>> simply clearing the nested translation capability?
> 
> You mean to tell no support of l2 translation via nest translation bit?
> But the nested translation is a different function with l2 translation
> even from guest view and nested translation only works requests with
> PASID (l1 translation).
> 
> Linux intel iommu driver enables l2 translation unconditionally and free 
> iommu instance when failed to enable l2 translation.

In which cases the wording of your description is confusing: Instead of
"Linux Intel IOMMU driver thinks l2 translation is always available when
VTD exits and fail to be loaded without l2 translation support ..." how
about using something closer to what you've replied with last?

Jan

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