[Xen-devel] [PATCH 2 of 9] x86/mm: Don't trigger unnecessary shadow scans on p2m entry update

[Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>]

 xen/arch/x86/mm/shadow/common.c  |  6 ++----
 xen/arch/x86/mm/shadow/multi.c   |  2 +-
 xen/arch/x86/mm/shadow/private.h |  7 +++++++
 3 files changed, 10 insertions(+), 5 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 47bfc8f8c0be -r 1b241f984167 xen/arch/x86/mm/shadow/common.c
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -2464,7 +2464,6 @@ int sh_remove_write_access_from_sl1p(str
 int sh_remove_all_mappings(struct vcpu *v, mfn_t gmfn)
 {
     struct page_info *page = mfn_to_page(gmfn);
-    int expected_count;
 
     /* Dispatch table for getting per-type functions */
     static const hash_callback_t callbacks[SH_type_unused] = {
@@ -2501,7 +2500,7 @@ int sh_remove_all_mappings(struct vcpu *
         ;
 
     perfc_incr(shadow_mappings);
-    if ( (page->count_info & PGC_count_mask) == 0 )
+    if ( __check_page_no_refs(page) )
         return 0;
 
     /* Although this is an externally visible function, we do not know
@@ -2517,8 +2516,7 @@ 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 )
+    if ( !__check_page_no_refs(page) )
     {
         /* Don't complain if we're in HVM and there are some extra mappings: 
          * The qemu helper process has an untyped mapping of this dom's RAM 
diff -r 47bfc8f8c0be -r 1b241f984167 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -4591,7 +4591,7 @@ int sh_rm_mappings_from_l1(struct vcpu *
         {
             (void) shadow_set_l1e(v, sl1e, shadow_l1e_empty(),
                                   p2m_invalid, sl1mfn);
-            if ( (mfn_to_page(target_mfn)->count_info & PGC_count_mask) == 0 )
+            if ( __check_page_no_refs(mfn_to_page(target_mfn)) )
                 /* This breaks us cleanly out of the FOREACH macro */
                 done = 1;
         }
diff -r 47bfc8f8c0be -r 1b241f984167 xen/arch/x86/mm/shadow/private.h
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -30,6 +30,7 @@
 #include <xen/domain_page.h>
 #include <asm/x86_emulate.h>
 #include <asm/hvm/support.h>
+#include <asm/atomic.h>
 
 #include "../mm-locks.h"
 
@@ -815,6 +816,12 @@ static inline unsigned long vtlb_lookup(
 }
 #endif /* (SHADOW_OPTIMIZATIONS & SHOPT_VIRTUAL_TLB) */
 
+static inline int __check_page_no_refs(struct page_info *page)
+{
+    unsigned long __count = read_atomic(&page->count_info);
+    return ( (__count & PGC_count_mask) ==
+             ((__count & PGC_allocated) ? 1 : 0) ); 
+}
 
 #endif /* _XEN_SHADOW_PRIVATE_H */
 

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