[Xen-devel] [PATCH 1/2] Revert "xen: properly account for _PAGE_NUMA during xen pte translations"

[David Vrabel <david.vrabel@xxxxxxxxxx>]

This reverts commit a9c8e4beeeb64c22b84c803747487857fe424b68.

PTEs in Xen PV guests must contain machine addresses if _PAGE_PRESENT
is set and pseudo-physical addresses is _PAGE_PRESENT is clear.

This is because during a domain save/restore (migration) the page
table entries are "canonicalised" and uncanonicalised". i.e., MFNs are
converted to PFNs during domain save so that on a restore the page
table entries may be rewritten with the new MFNs on the destination.

This change resulted in writing PTEs with MFNs if _PAGE_NUMA was set
but _PAGE_PRESENT was clear.  These PTEs would be migrated as-is which
would result in unexpected behaviour in the destination domain.
Either a) the MFN would be translated to the wrong PFN/page; b)
setting the _PAGE_PRESENT bit would clear the PTE because the MFN is
no longer owned by the domain; or c) the present bit would not get
set.

Symptoms include "Bad page" reports when unmapping after migrating a
domain.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Steven Noonan <steven@xxxxxxxxxxxxxx>
Cc: Elena Ufimtseva <ufimtseva@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>        [3.12+]
---
 arch/x86/include/asm/pgtable.h |   14 ++------------
 arch/x86/xen/mmu.c             |    4 ++--
 2 files changed, 4 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 5ad38ad..bbc8b12 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -445,20 +445,10 @@ static inline int pte_same(pte_t a, pte_t b)
        return a.pte == b.pte;
 }
 
-static inline int pteval_present(pteval_t pteval)
-{
-       /*
-        * Yes Linus, _PAGE_PROTNONE == _PAGE_NUMA. Expressing it this
-        * way clearly states that the intent is that protnone and numa
-        * hinting ptes are considered present for the purposes of
-        * pagetable operations like zapping, protection changes, gup etc.
-        */
-       return pteval & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_NUMA);
-}
-
 static inline int pte_present(pte_t a)
 {
-       return pteval_present(pte_flags(a));
+       return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE |
+                              _PAGE_NUMA);
 }
 
 #define pte_accessible pte_accessible
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 256282e..2423ef0 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -365,7 +365,7 @@ void xen_ptep_modify_prot_commit(struct mm_struct *mm, 
unsigned long addr,
 /* Assume pteval_t is equivalent to all the other *val_t types. */
 static pteval_t pte_mfn_to_pfn(pteval_t val)
 {
-       if (pteval_present(val)) {
+       if (val & _PAGE_PRESENT) {
                unsigned long mfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT;
                unsigned long pfn = mfn_to_pfn(mfn);
 
@@ -381,7 +381,7 @@ static pteval_t pte_mfn_to_pfn(pteval_t val)
 
 static pteval_t pte_pfn_to_mfn(pteval_t val)
 {
-       if (pteval_present(val)) {
+       if (val & _PAGE_PRESENT) {
                unsigned long pfn = (val & PTE_PFN_MASK) >> PAGE_SHIFT;
                pteval_t flags = val & PTE_FLAGS_MASK;
                unsigned long mfn;
-- 
1.7.2.5


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