[Xen-devel] [for-4.7 1/2] xen/arm: p2m: apply_p2m_changes: Do not undo more than necessary

[Julien Grall <julien.grall@xxxxxxx>]

Since commit 4b25423a "arch/arm: unmap partially-mapped memory regions",
Xen has been undoing the P2M mappings when an error occurred during
insertion or memory allocation.

The function apply_p2m_changes can work with region not-aligned to a
block size (2MB, 1G) or page size (4K). The mapping will be done by
splitting the region in a set of regions aligned to the size supported
by the page table.

The mapping of a region could fail when it is not possible to allocate
memory for an intermediate table (i.e a new or when shattering a block).

When the mapping is undone, the end of the region is computed using the
base address of the current region and the size of the failing level.
However the failing level may not be the leaf one, therefore unrelated
entries will be removed.

Fix it by removing the mapping from the start address up to the last
region that has been successfully mapped.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    This patch is a bug fix for Xen 4.7 and candidate for backporting
    up to Xen 4.5. Without this patch, Xen may undo mapping which are
    not part of the region mapped when memory allocation has failed.

    Note that Xen 4.7 has code to remove empty translation table (see
    commit de5162b "xen/arm: p2m: Remove translation table when it's
    empty"), however with this patch those tables will not be removed
    in case of failure. This will be fixed after the release as
    the change will be too intrusive for Xen 4.7.
---
 xen/arch/arm/p2m.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
index db21433..68c67b0 100644
--- a/xen/arch/arm/p2m.c
+++ b/xen/arch/arm/p2m.c
@@ -1189,13 +1189,10 @@ out:
     {
         BUG_ON(addr == end_gpaddr);
         /*
-         * addr keeps the address of the last successfully-inserted mapping,
-         * while apply_p2m_changes() considers an address range which is
-         * exclusive of end_gpaddr: add level_size to addr to obtain the
-         * right end of the range
+         * addr keeps the address of the end of the last successfully-inserted
+         * mapping.
          */
-        apply_p2m_changes(d, REMOVE,
-                          start_gpaddr, addr + level_sizes[level], orig_maddr,
+        apply_p2m_changes(d, REMOVE, start_gpaddr, addr, orig_maddr,
                           mattr, 0, p2m_invalid, d->arch.p2m.default_access);
     }
 
-- 
1.9.1


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