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 13:15
> 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 12:09, <Paul.Durrant@xxxxxxxxxx> wrote:
> >>  -----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?
> 
> Well, as soon as the function doesn't hand back MFNs anymore to
> HVM callers, no such assertion would be needed anymore either.
> 

So you'd prefer I add an ASSERTion that I'm going to remove as soon as I add a 
caller of the function?

  Paul

> Jan
> 


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