[PATCH v4 04/17] 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 aligned to unsigned long,
the single unsigned long location may cross two cache lines and
accessing the location by locked BTS/BTR introductions will trigger #AC.

To fix the split lock issue, align x86_capability to 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 unsigned long. But
this needs additional code changes. So we choose the simpler solution
by enforcing alignment using __aligned(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 33051436c864..eb8ae701ef65 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];
+	/* Unsigned long alignment to avoid split lock in atomic bitmap 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: */
-- 
2.7.4