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>

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index f1af718519195efcaf7f6c02f4b8069154123f99..25581dcb280ea8d3a9c8840cd70fcc726e10b76f 100644 (file)
--- 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;
 }