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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.10.14 at 13:37, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 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.

It can't be that difficult for Dom0 to track at which PFN is maps a
particular grant. And as long as Dom0 serializes properly wrt unmaps,
which GFN of the perhaps multiple ones it picks for doing the cache
flush doesn't even matter.

Jan


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