[PATCH v5 1/3] x86/cpufeatures: Enumerate #DB for bus lock detection

[Fenghua Yu <fenghua.yu@intel.com>]

A bus lock is acquired though either split locked access to
writeback (WB) memory or any locked access to non-WB memory. This is
typically >1000 cycles slower than an atomic operation within a cache
line. It also disrupts performance on other cores.

Some CPUs have ability to notify the kernel by an #DB trap after a user
instruction acquires a bus lock and is executed. This allows the kernel
to enforce user application throttling or mitigations. Both breakpoint
and bus lock can trigger the #DB trap in the same instruction and the
ordering of handling them is the kernel #DB handler's choice.

The CPU feature flag to be shown in /proc/cpuinfo will be "bus_lock_detect".

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
---
Change Log:
v5:
- Add "Both breakpoint and bus lock can trigger an #DB trap..." in the
  commit message (Thomas).

 arch/x86/include/asm/cpufeatures.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index cc96e26d69f7..faec3d92d09b 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -354,6 +354,7 @@
 #define X86_FEATURE_AVX512_VPOPCNTDQ	(16*32+14) /* POPCNT for vectors of DW/QW */
 #define X86_FEATURE_LA57		(16*32+16) /* 5-level page tables */
 #define X86_FEATURE_RDPID		(16*32+22) /* RDPID instruction */
+#define X86_FEATURE_BUS_LOCK_DETECT	(16*32+24) /* Bus Lock detect */
 #define X86_FEATURE_CLDEMOTE		(16*32+25) /* CLDEMOTE instruction */
 #define X86_FEATURE_MOVDIRI		(16*32+27) /* MOVDIRI instruction */
 #define X86_FEATURE_MOVDIR64B		(16*32+28) /* MOVDIR64B instruction */
-- 
2.30.2