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 10:03
> 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 10:53, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> From: Paul Durrant
> >> Sent: 12 September 2018 09:52
> >>
> >> > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> >> Behalf
> >> > Of Paul Durrant
> >> > Sent: 12 September 2018 09:45
> >> >
> >> > > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> > > Sent: 12 September 2018 09:44
> >> > >
> >> > > >>> On 12.09.18 at 10:31, <Paul.Durrant@xxxxxxxxxx> wrote:
> >> > > >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> > > >> Sent: 07 September 2018 12:11
> >> > > >>
> >> > > >> >>> On 23.08.18 at 11:47, <paul.durrant@xxxxxxxxxx> wrote:
> >> > > >> > This patch adds a new method to the VT-d IOMMU
> implementation
> >> to
> >> > > find
> >> > > >> the
> >> > > >> > MFN currently mapped by the specified BFN along with a
> wrapper
> >> > > function
> >> > > >> in
> >> > > >> > generic IOMMU code to call the implementation if it exists.
> >> > > >>
> >> > > >> For this to go in, I think the AMD side of it wants to also be
> >> > implemented.
> >> > > >>
> >> > > >> > --- a/xen/drivers/passthrough/iommu.c
> >> > > >> > +++ b/xen/drivers/passthrough/iommu.c
> >> > > >> > @@ -305,6 +305,17 @@ int iommu_unmap_page(struct domain
> *d,
> >> > > bfn_t
> >> > > >> bfn)
> >> > > >> >      return rc;
> >> > > >> >  }
> >> > > >> >
> >> > > >> > +int iommu_lookup_page(struct domain *d, bfn_t bfn, mfn_t
> *mfn,
> >> > > >> > +                      unsigned int *flags)
> >> > > >> > +{
> >> > > >> > +    const struct domain_iommu *hd = dom_iommu(d);
> >> > > >> > +
> >> > > >> > +    if ( !iommu_enabled || !hd->platform_ops )
> >> > > >> > +        return -EOPNOTSUPP;
> >> > > >> > +
> >> > > >> > +    return hd->platform_ops->lookup_page(d, bfn, mfn, flags);
> >> > > >> > +}
> >> > > >>
> >> > > >> Shouldn't this be restricted to PV guests? HVM ones aren't
> supposed
> >> > > >> to know about MFNs.
> >> > > >
> >> > > > Agreed, but I think this is the wrong level to be applying such a
> check:
> >> > > > iommu_map_page() is supplied an MFN regardless of whether the
> >> > domain
> >> > > is PV or
> >> > > > HVM, so I think it is reasonable for a lookup function to work in
> terms of
> >> > > > MFNs.
> >> > >
> >> > > I don't mind much where the check sits, but
> ASSERT(!is_hvm_domain()),
> >> > > if placed here, should not trigger.
> >> > >
> >> >
> >> > It will though. I'm going to need to use this function for HVM guests
> after
> >> > having done a GFN lookup.
> >>
> >> Sorry... I'm getting confused myself now. It won't fire because in my case
> the
> >> domain here will always by PV (because it is the not the domain owning
> the
> >> GFN). I still think this is the wrong level for such a check though, but 
> >> I'll put
> in
> >> the ASSERT.
> 
> And what would guarantee the ASSERT() to not trigger? As said, I'm
> fine with this being enforced at a different level, but it needs to be
> enforced somewhere.
> 
> > Actually, no I still don't think the ASSERT is correct. Why should we rule
> > out HVM guests being able to use PV-IOMMU?
> 
> 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?

  Paul

> Jan
> 


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