Messages in this thread Patch in this message | | | From | Fenghua Yu <> | Subject | [PATCH 05/15] x86/split_lock: Use non atomic set and clear bit instructions to clear cpufeature | Date | Mon, 14 May 2018 11:52:15 -0700 |
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When #AC exception is enabled for split locked accesses, clear_cpufeature() generates #AC exception because of atomic setting or clearing bits in non cache line aligned x86_capability depending on compiler.
But kernel clears cpufeatures only when a CPU boots up. Therefore, there is no racing condition when clear_cpufeature() is called and no need to atomically clear or set x86_capability.
To avoid #AC exception caused by split lock and get better performance, call non atomic __set_bit() and __clear_bit().
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> --- arch/x86/kernel/cpu/cpuid-deps.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c index 2c0bd38a44ab..0c02c6e44ea7 100644 --- a/arch/x86/kernel/cpu/cpuid-deps.c +++ b/arch/x86/kernel/cpu/cpuid-deps.c @@ -65,15 +65,15 @@ static const struct cpuid_dep cpuid_deps[] = { static inline void clear_feature(struct cpuinfo_x86 *c, unsigned int feature) { /* - * Note: This could use the non atomic __*_bit() variants, but the - * rest of the cpufeature code uses atomics as well, so keep it for - * consistency. Cleanup all of it separately. + * Because this code is only called during boot time and there + * is no need to be atomic, use non atomic __*_bit() to avoid + * #AC exception for split locked access. */ if (!c) { clear_cpu_cap(&boot_cpu_data, feature); - set_bit(feature, (unsigned long *)cpu_caps_cleared); + __set_bit(feature, (unsigned long *)cpu_caps_cleared); } else { - clear_bit(feature, (unsigned long *)c->x86_capability); + __clear_bit(feature, (unsigned long *)c->x86_capability); } } -- 2.5.0
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