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

At 12:59 +0100 on 02 Oct (1412251189), Jan Beulich wrote:
> >>> On 02.10.14 at 13:41, <tim@xxxxxxx> wrote:
> > Blargh.  Silly linux, forgetting its grant handles. :)
> > 
> > So after some IRL discussion with Stefano and IanC, I'm convinced that
> > the alternatives (linux maintaining per-map lookup tables to get back
> > from DMA address to grant handle) aren't going to work.  But we can
> > make this API a bit neater.  I think the design we came up with is:
> > 
> >  - the cache-flush hypercall becomes a grant-table op instead of a
> >    memop.
> >  - the argument becomes a 'dev_bus_addr' instead of an MFN.  That is,
> >    it must be the address returned in the dev_bus_addr field of a
> >    grant map operation.  If not, it will return EINVAL.
> Then at the very least let's allow for either a dev_bus_addr or
> a grant-ref (even if for the moment the latter may yield
> -EOPNOTSUPP in order to not spend more time on this than
> immediately necessary). That way guests remembering what
> they did a few microseconds back can do this in a more well
> behaved fashion.

Good idea.

Another thing that occurs to me: this should handle transitive grants
as well if possible.



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