[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>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

diff --git a/arch/x86_64/kernel/setup.c b/arch/x86_64/kernel/setup.c
index fc944b5e8f4a6050612a0c9220c713e2f47d3a51..013539fa53f01f60247ae163753aa8bbd07164cf 100644 (file)
--- a/arch/x86_64/kernel/setup.c
+++ b/arch/x86_64/kernel/setup.c
@@ -854,7 +854,10 @@ 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();