Re: [Xen-devel] [PATCH v3 0/3] map grant refs at pfn = mfn

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 1 Aug 2014, Julien Grall wrote:
> On 01/08/14 17:16, Thomas Leonard wrote:
> > On 1 August 2014 16:13, Stefano Stabellini
> > <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > > On Fri, 1 Aug 2014, Thomas Leonard wrote:
> > > > On 24/07/14 14:30, Stefano Stabellini wrote:
> > > > > Hi all,
> > > > > this patch series introduces a second p2m mapping of grant reference
> > > > > on
> > > > > ARM at guest physical address == machine address of the grant ref.  It
> > > > > is safe because dom0 is already mapped 1:1. We export
> > > > > XENFEAT_grant_map_identity to signal the guest that this second p2m
> > > > > mapping is
> > > > > available.
> > > > > 
> > > > > One reason for wanting the second p2m mapping is to avoid tracking mfn
> > > > > to pfn mappings in the guest kernel. Since the same mfn can be granted
> > > > > multiple times to the backend, finding the right pfn corresponding to
> > > > > a
> > > > > given mfn can be difficult and expensive. Providing a second mapping
> > > > > at
> > > > > a known address allow the kernel to access the page without knowing
> > > > > the
> > > > > pfn.
> > > > 
> > > > Is there a version of these patches for Xen 4.4 that I can test? The
> > > > restriction on duplicate pages is causing trouble for networking on
> > > > Mirage too
> > > > (http://roscidus.com/blog/blog/2014/07/28/my-first-unikernel/#tcp-retransmissions).
> > > 
> > > The backport is non-trivial because
> > > 552710b388630dfa461932940a998e120c42277d is missing from 4.4,
> > > nonetheless it wasn't too hard to port:
> > > 
> > >    git://xenbits.xen.org/people/sstabellini/xen-unstable.git
> > > grant_map_identity_4.4
> > 
> > Thanks. I merged it with the stable-4.4 branch (as that has some
> > useful fixes too), but it crashed for me when I started my Mirage
> > guest:
> 
> You have to drop the BUG_ON line 729.

Yes, you are right

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