From: Krzysztof Helt <krzysztof.h1@wp.pl>
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>
---
The second revision of this patch adds comment
after request from Jeremy Fitzhardinge.
I have tested the 6x86MX cpu with the CPUID
disabled. I have used linux-next tree (20080919)
and Yinghai Lu's patch:
x86: identify_cpu_without_cpuid v2
http://marc.info/?l=linux-kernel&m=122138380004347&w=2
The patch below is required to make the patch
above working correctly.
diff -urp linux-ref/arch/x86/kernel/cpu/common.c linux-mm/arch/x86/kernel/cpu/common.c
--- linux-ref/arch/x86/kernel/cpu/common.c 2008-09-30 23:07:08.000000000 +0200
+++ linux-mm/arch/x86/kernel/cpu/common.c 2008-09-30 23:12:19.098190815 +0200
@@ -124,18 +124,25 @@ static inline int flag_is_changeable_p(u
{
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;
}
--
