[PATCH] shadow: fix the fix for promotion/resync race.

[Gianluca Guida <gianluca.guida@eu.citrix.com>]

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Hello,

I've just realized that part of the fix sent to avoid the race between 
page promotion and resync contained a mistake.

This patch fixes it, and fixes also performance on 64bit guests by 
resyncing on validation only when needed.

Like the previous patch, this should be applied to Xen-3.3 as well.

Thanks,
Gianluca

Signed-off-by: Gianluca Guida <gianluca.guida@eu.citrix.com>


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diff -r ab0c1bdede53 xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c	Wed Nov 26 11:14:26 2008 +0000
+++ b/xen/arch/x86/mm/shadow/multi.c	Thu Nov 27 19:00:50 2008 +0000
@@ -1886,13 +1886,6 @@ static shadow_l1e_t * shadow_get_and_cre
         if ( r & SHADOW_SET_ERROR )
             return NULL;
 
-#if (SHADOW_OPTIMIZATIONS && SHOPT_OUT_OF_SYNC )
-        /* All pages walked are now pagetables. Safe to resync pages
-           in case level 4 or 3 shadows were set. */
-        if ( resync )
-            shadow_resync_all(v, 0);
-#endif
-
         /* This next line is important: in 32-on-PAE and 32-on-64 modes,
          * the guest l1 table has an 8k shadow, and we need to return
          * the right mfn of the pair. This call will set it for us as a
@@ -1900,6 +1893,14 @@ static shadow_l1e_t * shadow_get_and_cre
          * compiled out.) */
         (void) shadow_l1_index(sl1mfn, guest_l1_table_offset(gw->va));
     }
+
+#if (SHADOW_OPTIMIZATIONS && SHOPT_OUT_OF_SYNC )
+    /* All pages walked are now pagetables. Safe to resync pages
+       in case level 4 or 3 shadows were set. */
+    if ( resync )
+        shadow_resync_all(v, 0);
+#endif
+
     /* Now follow it down a level.  Guaranteed to succeed. */
     return sh_linear_l1_table(v) + shadow_l1_linear_offset(gw->va);
 }
@@ -2176,7 +2177,8 @@ static int validate_gl4e(struct vcpu *v,
             result |= SHADOW_SET_ERROR;
 
 #if (SHADOW_OPTIMIZATIONS && SHOPT_OUT_OF_SYNC )
-        shadow_resync_all(v, 0);
+        if ( mfn_valid(sl3mfn) )
+            shadow_resync_all(v, 0);
 #endif
     }
     l4e_propagate_from_guest(v, new_gl4e, sl3mfn, &new_sl4e, ft_prefetch);
@@ -2232,7 +2234,8 @@ static int validate_gl3e(struct vcpu *v,
             result |= SHADOW_SET_ERROR;
 
 #if (SHADOW_OPTIMIZATIONS && SHOPT_OUT_OF_SYNC )
-        shadow_resync_all(v, 0);
+        if ( mfn_valid(sl2mfn) )
+            shadow_resync_all(v, 0);
 #endif
     }
     l3e_propagate_from_guest(v, new_gl3e, sl2mfn, &new_sl3e, ft_prefetch);

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