Xen devel - Mar 2011 - [PATCH] x86: add volatile prefix for cpuid asm clauses

This is a bug fixing. So it needs go into 4.1.

x86: add volatile prefix for cpuid asm clauses

cpuid results are possible to be changed now. For example, changing CR4.OSXSAVE
bit or setting MSR XCR_XFEATURE_ENABLED_MASK may change XSAVE related cpuid
leave return values.

The volatile prefix is required to avoid the second cpuid calls following some
possible changing operations being optimized in incorrect way by compiler.

The sample bug is in xsave_init while debug=n. The second call to cpuid_count()
may be optimized and lead to a BUG_ON case while compare xsave_cntxt_size with
ebx.

Signed-off-by: Wei Gang <gang.wei@intel.com>

diff -r dc902ffeed73 xen/include/asm-x86/processor.h
--- a/xen/include/asm-x86/processor.h	Thu Mar 10 18:37:40 2011 +0000
+++ b/xen/include/asm-x86/processor.h	Wed Mar 16 02:30:49 2011 +0800
@@ -223,7 +223,7 @@ static always_inline void detect_ht(stru
  * resulting in stale register contents being returned.
  */
 #define cpuid(_op,_eax,_ebx,_ecx,_edx)          \
-    asm ( "cpuid"                               \
+    asm volatile ( "cpuid"                      \
           : "=a" (*(int *)(_eax)),              \
             "=b" (*(int *)(_ebx)),              \
             "=c" (*(int *)(_ecx)),              \
@@ -239,7 +239,7 @@ static inline void cpuid_count(
     unsigned int *ecx,
     unsigned int *edx)
 {
-    asm ( "cpuid"
+    asm volatile ( "cpuid"
           : "=a" (*eax), "=b" (*ebx), "=c"
(*ecx), "=d" (*edx)
           : "0" (op), "c" (count) );
 }
@@ -251,7 +251,7 @@ static always_inline unsigned int cpuid_
 {
     unsigned int eax;
 
-    asm ( "cpuid"
+    asm volatile ( "cpuid"
           : "=a" (eax)
           : "0" (op)
           : "bx", "cx", "dx" );
@@ -262,7 +262,7 @@ static always_inline unsigned int cpuid_
 {
     unsigned int eax, ebx;
 
-    asm ( "cpuid"
+    asm volatile ( "cpuid"
           : "=a" (eax), "=b" (ebx)
           : "0" (op)
           : "cx", "dx" );
@@ -273,7 +273,7 @@ static always_inline unsigned int cpuid_
 {
     unsigned int eax, ecx;
 
-    asm ( "cpuid"
+    asm volatile ( "cpuid"
           : "=a" (eax), "=c" (ecx)
           : "0" (op)
           : "bx", "dx" );
@@ -284,7 +284,7 @@ static always_inline unsigned int cpuid_
 {
     unsigned int eax, edx;
 
-    asm ( "cpuid"
+    asm volatile ( "cpuid"
           : "=a" (eax), "=d" (edx)
           : "0" (op)
           : "bx", "cx" );


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