[Xen-devel] [PATCH 05 of 14] Don't trigger unnecessary shadow scans on p2m entry update

[Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>]

 xen/arch/x86/mm/shadow/common.c |  4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)


When updating a p2m entry, the hypervisor scans all shadow pte's to find
mappings of that gfn and tear them down. This is avoided if the page count
reveals that there are no additional mappings. The current test ignores the
PGC_allocated flag and its effect on the page count.

Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
Signed-off-by: Adin Scannell <adin@xxxxxxxxxxx>

diff -r 93066bdc1e1c -r e8f0709af2b7 xen/arch/x86/mm/shadow/common.c
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -2501,7 +2501,8 @@ int sh_remove_all_mappings(struct vcpu *
         ;
 
     perfc_incr(shadow_mappings);
-    if ( (page->count_info & PGC_count_mask) == 0 )
+    expected_count = (page->count_info & PGC_allocated) ? 1 : 0;
+    if ( (page->count_info & PGC_count_mask) == expected_count )
         return 0;
 
     /* Although this is an externally visible function, we do not know
@@ -2517,7 +2518,6 @@ int sh_remove_all_mappings(struct vcpu *
     hash_foreach(v, callback_mask, callbacks, gmfn);
 
     /* If that didn't catch the mapping, something is very wrong */
-    expected_count = (page->count_info & PGC_allocated) ? 1 : 0;
     if ( (page->count_info & PGC_count_mask) != expected_count )
     {
         /* Don't complain if we're in HVM and there are some extra mappings: 

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