x86: do not allow to optimize flag_is_changeable_p() (rev. 2)

The flag_is_changeable_p() is used by
has_cpuid_p() which can return different results
in the code sequence below:

 if (!have_cpuid_p())
      identify_cpu_without_cpuid(c);

  /* cyrix could have cpuid enabled via c_identify()*/
  if (!have_cpuid_p())
      return;

Otherwise, the gcc 3.4.6 optimizes these two calls
into one which make the code not working correctly.

Cyrix cpus have the CPUID instruction enabled before
the second call to the have_cpuid_p() but
it is not detected due to the gcc optimization.
Thus the ARR registers (mtrr like) are not detected
on such a cpu.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 19 insertions, 12 deletions

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f1af71851919..25581dcb280e 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -124,18 +124,25 @@ static inline int flag_is_changeable_p(u32 flag)
 {
         u32 f1, f2;
 
-        asm("pushfl\n\t"
-            "pushfl\n\t"
-            "popl %0\n\t"
-            "movl %0,%1\n\t"
-            "xorl %2,%0\n\t"
-            "pushl %0\n\t"
-            "popfl\n\t"
-            "pushfl\n\t"
-            "popl %0\n\t"
-            "popfl\n\t"
-            : "=&r" (f1), "=&r" (f2)
-            : "ir" (flag));
+        /*
+         * Cyrix and IDT cpus allow disabling of CPUID
+         * so the code below may return different results
+         * when it is executed before and after enabling
+         * the CPUID. Add "volatile" to not allow gcc to
+         * optimize the subsequent calls to this function.
+         */
+        asm volatile ("pushfl\n\t"
+                      "pushfl\n\t"
+                      "popl %0\n\t"
+                      "movl %0,%1\n\t"
+                      "xorl %2,%0\n\t"
+                      "pushl %0\n\t"
+                      "popfl\n\t"
+                      "pushfl\n\t"
+                      "popl %0\n\t"
+                      "popfl\n\t"
+                      : "=&r" (f1), "=&r" (f2)
+                      : "ir" (flag));
 
         return ((f1^f2) & flag) != 0;
 }