Re: [Xen-devel] [PATCH 2/4] xen: introduce grant_map_exists

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 2 Oct 2014, Jan Beulich wrote:
> >>> On 02.10.14 at 12:42, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Thu, 2 Oct 2014, Tim Deegan wrote:
> >> At 11:02 +0100 on 02 Oct (1412244156), Stefano Stabellini wrote:
> >> > Check whether an mfn has been granted to a given domain on a target
> >> > grant table.
> >> 
> >> The implementation looks good but I'm concerned about the need for it,
> >> since linear scans of busy grant tables could get fairly expensive.
> >> 
> >> I see in patch 3/4 you add a hypercall to flush caches, taking an mfn
> >> range.  Should that not take a gfn range?  That would be the idiomatic
> >> interface, and the p2m lookup would tell you whether the guest had
> >> appropiate rights.
> >> 
> >> I suspect, from reading 0/4, that I'm missing something about the
> >> tangle of non-IOMMU dom0 operations on ARM. :)
> > 
> > The hypercall is going to be used to flush foreign pages grant mapped in
> > Dom0. Unfortunately dom0 doesn't know the gfn, only the dma address
> > (==mfn).
> 
> Don't grant maps get entered into the P2M as they get established?

Yes, they do. Still dom0 only has an mfn in its hands. Tracking the mfn
to pfn relationship in Linux has been tried unsuccessfully before. Not
only it is slow but it is also difficult as the same page can be granted
multiple times simultaneously.

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