Re: [PATCH v1 1/3] x86/cpufeatures: Add low performance CRC32C instruction CPU feature

On January 6, 2021 10:37:50 PM PST, Borislav Petkov [off-list ref] wrote:

On Thu, Jan 07, 2021 at 02:19:06PM +0800, Tony W Wang-oc wrote:

quoted

SSE4.2 on Zhaoxin CPUs are compatible with Intel. The presence of
CRC32C instruction is enumerated by CPUID.01H:ECX.SSE4_2[bit 20] = 1.
Some Zhaoxin CPUs declare support SSE4.2 instruction sets but their
CRC32C instruction are working with low performance.

Add a synthetic CPU flag to indicates that the CRC32C instruction is
not working as intended. This low performance CRC32C instruction flag
is depend on X86_FEATURE_XMM4_2.

Signed-off-by: Tony W Wang-oc <TonyWWang-oc@zhaoxin.com>
---
 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/kernel/cpu/cpuid-deps.c   | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h

b/arch/x86/include/asm/cpufeatures.h

quoted

index 84b8878..9e8151b 100644

--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h

@@ -292,6 +292,7 @@
 #define X86_FEATURE_FENCE_SWAPGS_KERNEL	(11*32+ 5) /* "" LFENCE in

kernel entry SWAPGS path */

quoted

 #define X86_FEATURE_SPLIT_LOCK_DETECT	(11*32+ 6) /* #AC for split

lock */

quoted

 #define X86_FEATURE_PER_THREAD_MBA	(11*32+ 7) /* "" Per-thread

Memory Bandwidth Allocation */

quoted

+#define X86_FEATURE_CRC32C		(11*32+ 8) /* "" Low performance CRC32C

instruction */

Didn't hpa say to create a BUG flag for it - X86_BUG...? Low
performance
insn sounds like a bug and not a feature to me.

And call it X86_BUG_CRC32C_SLOW or ..._UNUSABLE to denote what it
means.

Thx.

Yes, it should be a BUG due to the inclusion properties of BUG v FEATURE.
-- 
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