[PATCH] x86-64: Mark rdtsc as sync only for netburst, not for core2

On the Core2 cpus, the rdtsc instruction is not serializing (as defined
in the architecture reference since rdtsc exists) and due to the deep
speculation of these cores, it's possible that you can observe time go
backwards between cores due to this speculation. Since the kernel
already deals with this with the SYNC_RDTSC flag, the solution is
simple, only assume that the instruction is serializing on family 15...

The price one pays for this is a slightly slower gettimeofday (by a
dozen or two cycles), but that increase is quite small to pay for a
really-going-forward tsc counter.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andi Kleen <ak@suse.de>

diff --git a/arch/x86_64/kernel/setup.c b/arch/x86_64/kernel/setup.c
index 05eaca41802b97d88c6ca26c4210dac6ad385d49..6595a4ebe7f164430ee5d9d449b5ba6ba6bc0d77 100644 (file)
--- a/arch/x86_64/kernel/setup.c
+++ b/arch/x86_64/kernel/setup.c
@@ -860,7 +860,10 @@ static void __cpuinit init_intel(struct cpuinfo_x86 *c)
                set_bit(X86_FEATURE_CONSTANT_TSC, &c->x86_capability);
        if (c->x86 == 6)
                set_bit(X86_FEATURE_REP_GOOD, &c->x86_capability);
-       set_bit(X86_FEATURE_SYNC_RDTSC, &c->x86_capability);
+       if (c->x86 == 15)
+               set_bit(X86_FEATURE_SYNC_RDTSC, &c->x86_capability);
+       else
+               clear_bit(X86_FEATURE_SYNC_RDTSC, &c->x86_capability);
        c->x86_max_cores = intel_num_cpu_cores(c);
 
        srat_detect_node();