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Subject[PATCH 08/10] xen/mmu: Bugfix. Fill the top entry page with appropriate middle layer pointers.
If we swapped over from using an p2m_mid_identical to p2m_mid_missing
(earlier call to set_phys_to_machine) and then started going through the
PFNs in descending order to program a new MFN (balloon worker), we would
end up in this code path. At that point we would set up new page filled with
pointers to p2m_identity instead of p2m_missing. This had the disastrous
effect that get_phys_to_machine on that PFN would return an 1-1 mapping
instead of INVALID_P2M_ENTRY resulting in hitting a BUG check in balloon driver.

Signed-off-by: Konrad Rzeszutek Wilk <>
arch/x86/xen/mmu.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 92f4fec..a917439 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -480,7 +480,10 @@ static bool alloc_p2m(unsigned long pfn)
if (!mid)
return false;

- p2m_mid_init(mid, p2m_identity);
+ if (mid == p2m_mid_identity)
+ p2m_mid_init(mid, p2m_identity);
+ else
+ p2m_mid_init(mid, p2m_missing);

if (cmpxchg(top_p, mid_orig, mid) != mid_orig)

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