[PATCH 1/4] x86/cpufeatures: Enumerate #DB for bus lock detection

[Fenghua Yu]

A bus lock is acquired either through split locked access to
writeback (WB) memory or by using locks to uncacheable (UC) memory
(e.g. direct device assignment). 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.

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

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 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 dad350d42ecf..f375d9cb8123 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -352,6 +352,7 @@
 #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 */
-- 
2.29.2