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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 2 Oct 2014, Tim Deegan wrote:
> Hi,
> 
> 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).

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