[PATCH v3 0/3] xen/arm: fix "xen_add_mach_to_phys_entry: cannot add" problem

[Stefano Stabellini]

On Fri, 1 Aug 2014, David Vrabel wrote:
> On 01/08/14 15:55, Stefano Stabellini wrote:
> > Hi all,
> > Xen support in Linux for ARM and ARM64 suffers from lack of support for
> > multiple mfn to pfn mappings: whenever a frontend grants the same page
> > multiple times to the backend, the mfn to pfn accounting in
> > arch/arm/xen/p2m.c fails. The issue has become critical since v3.15,
> > when xen-netfront/xen-netback switched from grant copies to grant
> > mappings, therefore causing the issue to happen much more often.
> > 
> > Fixing the mfn to pfn accounting in p2m.c is difficult and expensive,
> > therefore we are looking for alternative solutions. One idea is avoiding
> > mfn to pfn conversions altogether. The only code path that needs them is
> > swiotlb-xen:unmap_page (and single_for_cpu and single_for_device).
> > 
> > To avoid mfn to pfn conversions we rely on a second p2m mapping done by
> > Xen (a separate patch series will be sent for Xen). In Linux we use it
> > to perform the cache maintenance operations without mfns conversions.
> > 
> > 
> > The Xen side patch series that introduces XENFEAT_grant_map_identity is:
> > http://marc.info/?l=xen-devel&m=140690433923855
> 
> If the Xen side is in master, you can apply this series to
> devel/for-linus-3.17

The Xen side patch finally went in. Unfortunately it's probably too late
for 3.17.

I have applied the latest version
(http://marc.info/?l=linux-arm-kernel&m=140751761520724&w=2) to
devel/for-linus-3.18.