From db8c33f8b5bea59d00ca12dcd6b65d01b1ea98ef Mon Sep 17 00:00:00 2001 From: Fenghua Yu Date: Mon, 16 Sep 2019 15:39:58 -0700 Subject: [PATCH] x86/cpu: Align the x86_capability array to size of unsigned long The x86_capability array in cpuinfo_x86 is of type u32 and thus is naturally aligned to 4 bytes. But, set_bit() and clear_bit() require the array to be aligned to size of unsigned long (i.e. 8 bytes on 64-bit systems). The array pointer is handed into atomic bit operations. If the access is not aligned to unsigned long then the atomic bit operations can end up crossing a cache line boundary, which causes the CPU to do a full bus lock as it can't lock both cache lines at once. The bus lock operation is heavy weight and can cause severe performance degradation. The upcoming #AC split lock detection mechanism will issue warnings for this kind of access. Force the alignment of the array to unsigned long. This avoids the massive code changes which would be required when converting the array data type to unsigned long. [ tglx: Rewrote changelog so it contains information WHY this is required ] Suggested-by: David Laight Suggested-by: Thomas Gleixner Signed-off-by: Fenghua Yu Signed-off-by: Tony Luck Signed-off-by: Thomas Gleixner Link: https://lkml.kernel.org/r/20190916223958.27048-4-tony.luck@intel.com --- arch/x86/include/asm/processor.h | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 6e0a3b43d027..c073534ca485 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -93,7 +93,15 @@ struct cpuinfo_x86 { __u32 extended_cpuid_level; /* Maximum supported CPUID level, -1=no CPUID: */ int cpuid_level; - __u32 x86_capability[NCAPINTS + NBUGINTS]; + /* + * Align to size of unsigned long because the x86_capability array + * is passed to bitops which require the alignment. Use unnamed + * union to enforce the array is aligned to size of unsigned long. + */ + union { + __u32 x86_capability[NCAPINTS + NBUGINTS]; + unsigned long x86_capability_alignment; + }; char x86_vendor_id[16]; char x86_model_id[64]; /* in KB - valid for CPUS which support this call: */