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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 1 Aug 2014, Thomas Leonard wrote:
> On 1 August 2014 17:25, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > 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
> 
> This one too?
> 
> (XEN) Xen BUG at grant_table.c:946
> (XEN) CPU1: Unexpected Trap: Undefined Instruction

Yep, sorry.

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