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

["Lan, Tianyu" <tianyu.lan@xxxxxxxxx>]

On 10/26/2016 5:39 PM, Jan Beulich wrote:
> > > > 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?
Sorry for my pool English. Will update.

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