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

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.

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

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