Re: [Xen-devel] [PATCH v6 08/14] vtd: add lookup_page method to iommu_ops

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 12 September 2018 11:08
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: George Dunlap <George.Dunlap@xxxxxxxxxx>; Kevin Tian
> <kevin.tian@xxxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: RE: [PATCH v6 08/14] vtd: add lookup_page method to iommu_ops
> 
> >>> On 12.09.18 at 11:30, <Paul.Durrant@xxxxxxxxxx> wrote:
> >>  -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: 12 September 2018 10:21
> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> >> Cc: George Dunlap <George.Dunlap@xxxxxxxxxx>; Kevin Tian
> >> <kevin.tian@xxxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> >> Subject: RE: [PATCH v6 08/14] vtd: add lookup_page method to
> iommu_ops
> >>
> >> >>> On 12.09.18 at 11:15, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> >>  -----Original Message-----
> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> Sent: 12 September 2018 10:13
> >> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> >> >> Cc: George Dunlap <George.Dunlap@xxxxxxxxxx>; Kevin Tian
> >> >> <kevin.tian@xxxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> >> >> Subject: RE: [PATCH v6 08/14] vtd: add lookup_page method to
> >> iommu_ops
> >> >>
> >> >> >>> On 12.09.18 at 11:05, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> >> Sent: 12 September 2018 10:03
> >> >> >>
> >> >> >> A HVM guest using the PV IOMMU is quite fine, but it shouldn't talk
> to
> >> >> >> it in terms of MFNs.
> >> >> >>
> >> >> >
> >> >> > Well, it has to talk MFNs at some level, surely? The output of the
> >> IOMMU is
> >> >> > not subject to EPT/NPT, right?
> >> >>
> >> >> Yes to the second question, but no to the first: The GFN -> MFN
> >> translation
> >> >> should still be done inside Xen in the HVM case, imo (in the course of
> >> >> manufacturing the PTE).
> >> >
> >> > Indeed. This function is very much internal to Xen (it's simply an
> >> > abstraction on top of a vendor implementation), so why should it not
> work
> >> in
> >> > terms of MFNs?
> >>
> >> Because "MFN" is a concept a HVM guest is not knowing about, or
> >> supposed to be knowing. The only time where (part of) it might
> >> legitimately (have to) know is when it comes to managing the host
> >> (including any guests), i.e. in the tool stack of a PVH Dom0.
> >
> > Ok. So consider validating a PV-IOMMU unmap request from an HVM
> guest. It
> > passes in a DFN and a GFN  belonging to itself. Now Xen needs to figure out
> > whether that BFN actually maps to the GFN. It can look up the MFN backing
> the
> > GFN (from the p2m). How does Xen now validate it if it cannot lookup what
> MFN
> > is actually present in the PTE referenced by the DFN?
> 
> I'm afraid I don't understand: The passed in GFN gets translated
> to an MFN using a p2m lookup. The passed in DFN (which aiui ought
> to match the GFN anyway on x86) gets translated to an MFN using
> an IOMMU page table lookup. The resulting two MFNs have to
> match for the request to be valid.
> 

Quite. So how does that work if iommu_lookup_page() is ASSERTing that the 
domain in question is not HVM?

  Paul

> Jan
> 


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