[tip: x86/splitlock] x86/cpufeatures: Enumerate #DB for bus lock detection

[tip-bot2 for Fenghua Yu]

The following commit has been merged into the x86/splitlock branch of tip:

Commit-ID:     f21d4d3b97a8603567e5d4250bd75e8ebbd520af
Gitweb:        https://git.kernel.org/tip/f21d4d3b97a8603567e5d4250bd75e8ebbd520af
Author:        Fenghua Yu <fenghua.yu@xxxxxxxxx>
AuthorDate:    Mon, 22 Mar 2021 13:53:23 
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sun, 28 Mar 2021 22:52:14 +02:00

x86/cpufeatures: Enumerate #DB for bus lock detection

A bus lock is acquired through either a 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 the ability to notify the kernel by a #DB trap after a user
instruction acquires a bus lock and is executed. This allows the kernel to
enforce user application throttling or mitigation. 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@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Link: https://lore.kernel.org/r/20210322135325.682257-2-fenghua.yu@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 cc96e26..faec3d9 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 */