[PATCH v8 04/15] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

[Fenghua Yu]

set_cpu_cap() calls locked BTS and clear_cpu_cap() calls locked BTR to
operate on bitmap defined in x86_capability.

Locked BTS/BTR accesses a single unsigned long location. In 64-bit mode,
the location is at:
base address of x86_capability + (bit offset in x86_capability / 64) * 8

Since base address of x86_capability may not be aligned to unsigned long,
the single unsigned long location may cross two cache lines and
accessing the location by locked BTS/BTR introductions will cause
split lock.

To fix the split lock issue, align x86_capability to size of unsigned long
so that the location will be always within one cache line.

Changing x86_capability's type to unsigned long may also fix the issue
because x86_capability will be naturally aligned to size of unsigned long.
But this needs additional code changes. So choose the simpler solution
by setting the array's alignment to size of unsigned long.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/include/asm/processor.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 2bb3a648fc12..7c62b9ad6e5a 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -93,7 +93,9 @@ struct cpuinfo_x86 {
 	__u32			extended_cpuid_level;
 	/* Maximum supported CPUID level, -1=no CPUID: */
 	int			cpuid_level;
-	__u32			x86_capability[NCAPINTS + NBUGINTS];
+	/* Aligned to size of unsigned long to avoid split lock in atomic ops */
+	__u32			x86_capability[NCAPINTS + NBUGINTS]
+				__aligned(sizeof(unsigned long));
 	char			x86_vendor_id[16];
 	char			x86_model_id[64];
 	/* in KB - valid for CPUS which support this call: */
-- 
2.19.1