Re: [Xen-devel] [PATCH v2 0/3] xen/arm: fix "xen_add_mach_to_phys_entry: cannot add" problem

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 8 Jul 2014, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 08, 2014 at 04:40:46PM +0100, 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).
> 
> I seem to have lost track of that patch? Or is it in now in the kernel?
> 
> Could you include the git commit id or URL for it in the cover letter please?

I take that you are asking the id of the commit that introduced the
function calls that need mfn to pfn conversions in swiotlb-xen, right?
The commit that introduced them is:

commit 6cf054636261ca5c88f3c2984058d51f927b8a2e
Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date:   Fri Oct 25 10:33:25 2013 +0000

    swiotlb-xen: use xen_dma_map/unmap_page, xen_dma_sync_single_for_cpu/device
     
xen_dma_unmap_page on x86 doesn't do anything but on ARM performs
important cache invalidate operations for non-dma-coherent devices.



> Thanks
> > 
> > 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.
> > 
> > 
> > Changes in v2:
> > - introduce XENFEAT_grant_map_11;
> > - remeber the ptep corresponding to scratch pages so that we don't need
> > to calculate it again every time;
> > - do not acutally unmap the page on xen_mm32_unmap;
> > - properly account preempt_enable/disable;
> > - do not check for mfn in xen_add_phys_to_mach_entry.
> > 
> > 
> > Stefano Stabellini (3):
> >       xen/arm: introduce XENFEAT_grant_map_11
> >       xen/arm: reimplement xen_dma_unmap_page & friends
> >       xen/arm: remove mach_to_phys rbtree
> > 
> >  arch/arm/include/asm/xen/page-coherent.h |   25 ++--
> >  arch/arm/include/asm/xen/page.h          |    9 --
> >  arch/arm/xen/Makefile                    |    2 +-
> >  arch/arm/xen/enlighten.c                 |    6 +
> >  arch/arm/xen/mm32.c                      |  202 
> > ++++++++++++++++++++++++++++++
> >  arch/arm/xen/p2m.c                       |   66 +---------
> >  include/xen/interface/features.h         |    3 +
> >  7 files changed, 220 insertions(+), 93 deletions(-)
> >  create mode 100644 arch/arm/xen/mm32.c
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel
> 

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