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 18:01, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > 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.
> 
> It seems that removing this makes it not compile:
> 
> arm-linux-gnueabihf-ld    -EL  -T xen.lds -N prelink.o \
>             /xen-arm-builder/xen/xen/common/symbols-dummy.o -o
> /xen-arm-builder/xen/xen/.xen-syms.0
> prelink.o: In function `__gnttab_unmap_common':
> /xen-arm-builder/xen/xen/common/grant_table.c:952: undefined reference
> to `iommu_unmap_page'
> /xen-arm-builder/xen/xen/common/grant_table.c:959: undefined reference
> to `iommu_map_page'
> prelink.o: In function `__gnttab_map_grant_ref':
> /xen-arm-builder/xen/xen/common/grant_table.c:739: undefined reference
> to `iommu_map_page'
> /xen-arm-builder/xen/xen/common/grant_table.c:750: undefined reference
> to `iommu_map_page'
> arm-linux-gnueabihf-ld: /xen-arm-builder/xen/xen/.xen-syms.0: hidden
> symbol `iommu_unmap_page' isn't defined
> arm-linux-gnueabihf-ld: final link failed: Bad value
> 
> Maybe the BUG_ON made this code unreachable before?

Checkout the branch grant_map_identity_4.4_2

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