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Subject[PATCH v1] x86/fpu: Remove opmask state from avx512_timestamp check
Date
This commit creates a new mask, XFEATURE_MASK_ZMM, to test against
xfeatures for conditionally updating the axx512_timestamp.

Based on the comments, the avx512 state is meant to track when the
state would cause frequencey throttling. The opmasks (k0-k7) do not
cause frequency throttling, so they don't make sense to include.

The current implementation, as well as the old, still does have a
false positive on ymm16-ymm31 and xmm16-31 because
XFEATURE_MASK_Hi16_ZMM includes them.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
---
Issue is reproducible with the following code on x86_64:

```
.global _start
.text
_start:
korq %k0, %k0, %k0

loop:
jmp loop


movl $60, %eax
xorl %edi, %edi
syscall
```

Pretending run as executable named "foo":

$> cat /proc/$(pidof foo)/arch_status


This should yield -1 as no frequency changing AVX512 instructions
where used but instead tracks the process.

Note there still is a false positive with ymm16-ymm31 and xmm16-xmm31
but since there is no state to distinguish between there use and
zmm16-31 that seems inevitable.


arch/x86/include/asm/fpu/types.h | 2 ++
arch/x86/kernel/fpu/core.c | 4 +++-
2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index f5a38a5f3ae1..a4816fa7d541 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -141,6 +141,8 @@ enum xfeature {
#define XFEATURE_MASK_AVX512 (XFEATURE_MASK_OPMASK \
| XFEATURE_MASK_ZMM_Hi256 \
| XFEATURE_MASK_Hi16_ZMM)
+#define XFEATURE_MASK_ZMM (XFEATURE_MASK_ZMM_Hi256 \
+ | XFEATURE_MASK_Hi16_ZMM)

#define FIRST_EXTENDED_XFEATURE XFEATURE_YMM

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 7ada7bd03a32..342620a2e8ef 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -104,8 +104,10 @@ void save_fpregs_to_fpstate(struct fpu *fpu)
/*
* AVX512 state is tracked here because its use is
* known to slow the max clock speed of the core.
+ * Note: This has a false positive on Hi16 ymm and
+ * xmm registers.
*/
- if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
+ if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_ZMM)
fpu->avx512_timestamp = jiffies;
return;
}
--
2.25.1
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