From 94f6bac1058fd59a8bd472d18c4b77f220d930b0 Mon Sep 17 00:00:00 2001 From: Krzysztof Helt Date: Tue, 30 Sep 2008 23:17:51 +0200 Subject: 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 Signed-off-by: Ingo Molnar --- arch/x86/kernel/cpu/common.c | 31 +++++++++++++++++++------------ 1 file changed, 19 insertions(+), 12 deletions(-) (limited to 'arch') diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index f1af7185191..25581dcb280 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; } -- cgit v1.2.3