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[Xen-devel] [PATCH 2/4] xen/mm: zero PTEs for non-present MFNs in the initial page table

From: David Vrabel <david.vrabel@xxxxxxxxxx>

When constructing the initial page tables, if the MFN for a usable PFN
is missing in the p2m then that frame is initially ballooned out.  In
this case, zero the PTE (as in decrease_reservation() in

This is obviously safe instead of having an valid PTE with an MFN of

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
 arch/x86/xen/mmu.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 3f1783a..27336df 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1432,6 +1432,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
  * Init-time set_pte while constructing initial pagetables, which
  * doesn't allow RO page table pages to be remapped RW.
+ * If there is no MFN for this PFN then this page is initially
+ * ballooned out so clear the PTE (as in decrease_reservation() in
+ * drivers/xen/balloon.c).
+ *
  * Many of these PTE updates are done on unpinned and writable pages
  * and doing a hypercall for these is unnecessary and expensive.  At
  * this point it is not possible to tell if a page is pinned or not,
@@ -1440,7 +1444,10 @@ static pte_t __init mask_rw_pte(pte_t *ptep, pte_t pte)
 static void __init xen_set_pte_init(pte_t *ptep, pte_t pte)
-       pte = mask_rw_pte(ptep, pte);
+       if (pte_mfn(pte) != INVALID_P2M_ENTRY)
+               pte = mask_rw_pte(ptep, pte);
+       else
+               pte = __pte_ma(0);
        native_set_pte(ptep, pte);

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