[PATCH 0/6] x86: add missing vzeroupper instructions

[PATCH 0/6] x86: add missing vzeroupper instructions

[Eric Biggers]

Assembly code using YMM or ZMM registers is supposed to end with the
vzeroupper instruction in order to avoid degrading the performance of
any later SSE code.  Since this only affects performance and not
correctness, it is sometimes overlooked.  Most kernel code does it
correctly, but a few cases were missed.  This series fixes them.

It should be easiest to take the full series through the x86 tree.

Eric Biggers (6):
  xor: add missing vzeroupper to AVX code
  raid6: add missing vzeroupper to AVX2 code
  raid6: add missing vzeroupper to AVX-512 code
  crypto: x86/aria - add missing vzeroupper in AVX2 code
  crypto: x86/aria - add missing vzeroupper in AVX-512 code
  netfilter: nft_set_pipapo_avx2: add missing vzeroupper

 arch/x86/crypto/aria-aesni-avx2-asm_64.S  |  6 ++++++
 arch/x86/crypto/aria-gfni-avx512-asm_64.S |  3 +++
 lib/raid/raid6/x86/avx2.c                 |  6 ++++++
 lib/raid/raid6/x86/avx512.c               |  6 ++++++
 lib/raid/raid6/x86/recov_avx2.c           |  2 ++
 lib/raid/raid6/x86/recov_avx512.c         |  2 ++
 lib/raid/xor/x86/xor-avx.c                |  1 +
 net/netfilter/nft_set_pipapo_avx2.c       | 17 ++++++++---------
 8 files changed, 34 insertions(+), 9 deletions(-)


base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
-- 
2.55.0

[PATCH 1/6] xor: add missing vzeroupper to AVX code

[Eric Biggers]

Since the AVX optimized XOR code uses YMM registers, execute vzeroupper
before returning from it.  This is needed to avoid degrading the
performance of any later SSE code that may happen to be executed.

Fixes: ea4d26ae24e5 ("raid5: add AVX optimized RAID5 checksumming")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 lib/raid/xor/x86/xor-avx.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/raid/xor/x86/xor-avx.c b/lib/raid/xor/x86/xor-avx.c
index f7777d7aa269..95b21e7225e8 100644
--- a/lib/raid/xor/x86/xor-avx.c
+++ b/lib/raid/xor/x86/xor-avx.c
@@ -147,6 +147,7 @@ static void xor_gen_avx(void *dest, void **srcs, unsigned int src_cnt,
 {
 	kernel_fpu_begin();
 	xor_gen_avx_inner(dest, srcs, src_cnt, bytes);
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
-- 
2.55.0

[PATCH 2/6] raid6: add missing vzeroupper to AVX2 code

[Eric Biggers]

Since the AVX2 optimized RAID6 code uses YMM registers, execute
vzeroupper before returning from it.  This is needed to avoid degrading
the performance of any later SSE code that may happen to be executed.

Fixes: 2c935842bdb4 ("lib/raid6: Add AVX2 optimized gen_syndrome functions")
Fixes: 7056741fd9fc ("lib/raid6: Add AVX2 optimized recovery functions")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 lib/raid/raid6/x86/avx2.c       | 6 ++++++
 lib/raid/raid6/x86/recov_avx2.c | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/lib/raid/raid6/x86/avx2.c b/lib/raid/raid6/x86/avx2.c
index 7d829c669ea7..3cc2fe7ac42c 100644
--- a/lib/raid/raid6/x86/avx2.c
+++ b/lib/raid/raid6/x86/avx2.c
@@ -67,6 +67,7 @@ static void raid6_avx21_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -115,6 +116,7 @@ static void raid6_avx21_xor_syndrome(int disks, int start, int stop,
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -175,6 +177,7 @@ static void raid6_avx22_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -243,6 +246,7 @@ static void raid6_avx22_xor_syndrome(int disks, int start, int stop,
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -334,6 +338,7 @@ static void raid6_avx24_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -444,6 +449,7 @@ static void raid6_avx24_xor_syndrome(int disks, int start, int stop,
 		asm volatile("vmovntdq %%ymm14,%0" : "=m" (q[d+96]));
 	}
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
diff --git a/lib/raid/raid6/x86/recov_avx2.c b/lib/raid/raid6/x86/recov_avx2.c
index a714a780a2d8..820871046e30 100644
--- a/lib/raid/raid6/x86/recov_avx2.c
+++ b/lib/raid/raid6/x86/recov_avx2.c
@@ -176,6 +176,7 @@ static void raid6_2data_recov_avx2(int disks, size_t bytes, int faila,
 #endif
 	}
 
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -293,6 +294,7 @@ static void raid6_datap_recov_avx2(int disks, size_t bytes, int faila,
 #endif
 	}
 
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
-- 
2.55.0

[PATCH 3/6] raid6: add missing vzeroupper to AVX-512 code

[Eric Biggers]

Since the AVX-512 optimized RAID6 code uses ZMM registers, execute
vzeroupper before returning from it.  This is needed to avoid degrading
the performance of any later SSE code that may happen to be executed.

Fixes: e0a491c12968 ("lib/raid6: Add AVX512 optimized gen_syndrome functions")
Fixes: 13c520b2993c ("lib/raid6: Add AVX512 optimized recovery functions")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 lib/raid/raid6/x86/avx512.c       | 6 ++++++
 lib/raid/raid6/x86/recov_avx512.c | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/lib/raid/raid6/x86/avx512.c b/lib/raid/raid6/x86/avx512.c
index e671eb5bde63..772bfc4af6df 100644
--- a/lib/raid/raid6/x86/avx512.c
+++ b/lib/raid/raid6/x86/avx512.c
@@ -78,6 +78,7 @@ static void raid6_avx5121_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -137,6 +138,7 @@ static void raid6_avx5121_xor_syndrome(int disks, int start, int stop,
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -208,6 +210,7 @@ static void raid6_avx5122_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -292,6 +295,7 @@ static void raid6_avx5122_xor_syndrome(int disks, int start, int stop,
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -396,6 +400,7 @@ static void raid6_avx5124_gen_syndrome(int disks, size_t bytes, void **ptrs)
 	}
 
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -529,6 +534,7 @@ static void raid6_avx5124_xor_syndrome(int disks, int start, int stop,
 			       "m" (q[d+128]), "m" (q[d+192]));
 	}
 	asm volatile("sfence" : : : "memory");
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 const struct raid6_calls raid6_avx512x4 = {
diff --git a/lib/raid/raid6/x86/recov_avx512.c b/lib/raid/raid6/x86/recov_avx512.c
index ec72d5a30c01..299a3f044d61 100644
--- a/lib/raid/raid6/x86/recov_avx512.c
+++ b/lib/raid/raid6/x86/recov_avx512.c
@@ -211,6 +211,7 @@ static void raid6_2data_recov_avx512(int disks, size_t bytes, int faila,
 #endif
 	}
 
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
@@ -353,6 +354,7 @@ static void raid6_datap_recov_avx512(int disks, size_t bytes, int faila,
 #endif
 	}
 
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 }
 
-- 
2.55.0

[PATCH 4/6] crypto: x86/aria - add missing vzeroupper in AVX2 code

[Eric Biggers]

Since the AVX2 optimized ARIA code uses YMM registers, execute
vzeroupper before returning from it.  This is needed to avoid degrading
the performance of any later SSE code that may happen to be executed.

Fixes: 37d8d3ae7a58 ("crypto: x86/aria - implement aria-avx2")
Cc: stable@vger.kernel.org
Cc: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 arch/x86/crypto/aria-aesni-avx2-asm_64.S | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/crypto/aria-aesni-avx2-asm_64.S b/arch/x86/crypto/aria-aesni-avx2-asm_64.S
index ed53d4f46bd7..fda8cb8a99a8 100644
--- a/arch/x86/crypto/aria-aesni-avx2-asm_64.S
+++ b/arch/x86/crypto/aria-aesni-avx2-asm_64.S
@@ -982,6 +982,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_encrypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_encrypt_32way)
@@ -1007,6 +1008,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_decrypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_decrypt_32way)
@@ -1209,6 +1211,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_ctr_crypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %r10);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_ctr_crypt_32way)
@@ -1359,6 +1362,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_gfni_encrypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_gfni_encrypt_32way)
@@ -1384,6 +1388,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_gfni_decrypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_gfni_decrypt_32way)
@@ -1428,6 +1433,7 @@ SYM_TYPED_FUNC_START(aria_aesni_avx2_gfni_ctr_crypt_32way)
 		     %ymm8, %ymm9, %ymm10, %ymm11, %ymm12, %ymm13, %ymm14,
 		     %ymm15, %r10);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_aesni_avx2_gfni_ctr_crypt_32way)
-- 
2.55.0

[PATCH 5/6] crypto: x86/aria - add missing vzeroupper in AVX-512 code

[Eric Biggers]

Since the AVX-512 optimized ARIA code uses ZMM registers, execute
vzeroupper before returning from it.  This is needed to avoid degrading
the performance of any later SSE code that may happen to be executed.

Fixes: c970d42001f2 ("crypto: x86/aria - implement aria-avx512")
Cc: stable@vger.kernel.org
Cc: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 arch/x86/crypto/aria-gfni-avx512-asm_64.S | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/crypto/aria-gfni-avx512-asm_64.S b/arch/x86/crypto/aria-gfni-avx512-asm_64.S
index 860887e5d02e..ca83eb126e06 100644
--- a/arch/x86/crypto/aria-gfni-avx512-asm_64.S
+++ b/arch/x86/crypto/aria-gfni-avx512-asm_64.S
@@ -800,6 +800,7 @@ SYM_TYPED_FUNC_START(aria_gfni_avx512_encrypt_64way)
 		     %zmm9, %zmm8, %zmm11, %zmm10, %zmm12, %zmm13, %zmm14,
 		     %zmm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_gfni_avx512_encrypt_64way)
@@ -825,6 +826,7 @@ SYM_TYPED_FUNC_START(aria_gfni_avx512_decrypt_64way)
 		     %zmm9, %zmm8, %zmm11, %zmm10, %zmm12, %zmm13, %zmm14,
 		     %zmm15, %rax);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_gfni_avx512_decrypt_64way)
@@ -966,6 +968,7 @@ SYM_TYPED_FUNC_START(aria_gfni_avx512_ctr_crypt_64way)
 		     %zmm9, %zmm8, %zmm11, %zmm10, %zmm12, %zmm13, %zmm14,
 		     %zmm15, %r10);
 
+	vzeroupper;
 	FRAME_END
 	RET;
 SYM_FUNC_END(aria_gfni_avx512_ctr_crypt_64way)
-- 
2.55.0

[PATCH 6/6] netfilter: nft_set_pipapo_avx2: add missing vzeroupper

[Eric Biggers]

Since pipapo_get_avx2() uses YMM registers, execute vzeroupper before
returning from it.  This is needed to avoid degrading the performance of
any later SSE code that may happen to be executed.

Fixes: 7400b063969b ("nft_set_pipapo: Introduce AVX2-based lookup implementation")
Cc: stable@vger.kernel.org
Cc: Florian Westphal <fw@strlen.de>
Cc: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 net/netfilter/nft_set_pipapo_avx2.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/net/netfilter/nft_set_pipapo_avx2.c b/net/netfilter/nft_set_pipapo_avx2.c
index b3f105520a85..21f5be68c703 100644
--- a/net/netfilter/nft_set_pipapo_avx2.c
+++ b/net/netfilter/nft_set_pipapo_avx2.c
@@ -1134,6 +1134,7 @@ struct nft_pipapo_elem *pipapo_get_avx2(const struct nft_pipapo_match *m,
 	struct nft_pipapo_scratch *scratch;
 	const struct nft_pipapo_field *f;
 	unsigned long *res, *fill, *map;
+	struct nft_pipapo_elem *e;
 	bool map_index;
 	int ret = 0;
 	int i;
@@ -1207,14 +1208,11 @@ struct nft_pipapo_elem *pipapo_get_avx2(const struct nft_pipapo_match *m,
 next_match:
 		if (ret < 0) {
 			scratch->map_index = map_index;
-			kernel_fpu_end();
-			__local_unlock_nested_bh(&scratch->bh_lock);
-			return NULL;
+			e = NULL;
+			goto out;
 		}
 
 		if (last) {
-			struct nft_pipapo_elem *e;
-
 			e = f->mt[ret].e;
 			if (unlikely(__nft_set_elem_expired(&e->ext, tstamp) ||
 				     !nft_set_elem_active(&e->ext, genmask))) {
@@ -1224,9 +1222,7 @@ struct nft_pipapo_elem *pipapo_get_avx2(const struct nft_pipapo_match *m,
 			}
 
 			scratch->map_index = map_index;
-			kernel_fpu_end();
-			__local_unlock_nested_bh(&scratch->bh_lock);
-			return e;
+			goto out;
 		}
 
 		map_index = !map_index;
@@ -1234,9 +1230,12 @@ struct nft_pipapo_elem *pipapo_get_avx2(const struct nft_pipapo_match *m,
 		data += NFT_PIPAPO_GROUPS_PADDED_SIZE(f);
 	}
 
+	e = NULL;
+out:
+	asm volatile("vzeroupper");
 	kernel_fpu_end();
 	__local_unlock_nested_bh(&scratch->bh_lock);
-	return NULL;
+	return e;
 }
 
 /**
-- 
2.55.0

Re: [PATCH 6/6] netfilter: nft_set_pipapo_avx2: add missing vzeroupper

[Stefano Brivio]

Eric, thanks for taking care of this.

The patch looks good to me, I just have two questions:

On Sat, 15 Aug 2026 13:57:50 -0700
Eric Biggers <ebiggers@kernel.org> wrote:

> Since pipapo_get_avx2() uses YMM registers, execute vzeroupper before
> returning from it.  This is needed to avoid degrading the performance of
> any later SSE code that may happen to be executed.

Out of curiosity: was this prompted by some observed latency spike in
execution of SSE code, or it's just meant to satisfy the recommendation
from AMD and Intel to use it while transitioning from AVX to SSE modes?

> Fixes: 7400b063969b ("nft_set_pipapo: Introduce AVX2-based lookup implementation")
> Cc: stable@vger.kernel.org

Is this really stable material? Skipping vzeroupper might have a
performance impact, but it's not an issue for correctness.

The main reason why I'm asking is that, while vzeroupper might look
harmless and obviously safe, it actually caused CVE-2023-20593
("ZenBleed") on AMD Zen 2.

I expect systems receiving stable kernel updates to also run the
patched microcode by now, so I'm not overly concerned in any case.

-- 
Stefano

Re: [PATCH 6/6] netfilter: nft_set_pipapo_avx2: add missing vzeroupper

[Eric Biggers]

On Sun, Aug 16, 2026 at 12:38:41PM +0200, Stefano Brivio wrote:
> Eric, thanks for taking care of this.
> 
> The patch looks good to me, I just have two questions:
> 
> On Sat, 15 Aug 2026 13:57:50 -0700
> Eric Biggers <ebiggers@kernel.org> wrote:
> 
> > Since pipapo_get_avx2() uses YMM registers, execute vzeroupper before
> > returning from it.  This is needed to avoid degrading the performance of
> > any later SSE code that may happen to be executed.
> 
> Out of curiosity: was this prompted by some observed latency spike in
> execution of SSE code, or it's just meant to satisfy the recommendation
> from AMD and Intel to use it while transitioning from AVX to SSE modes?

This one was found by code review.  But the latency spike has been
observed in other cases with missing vzeroupper, so it's definitely a
real effect at least on some CPUs.

> > Fixes: 7400b063969b ("nft_set_pipapo: Introduce AVX2-based lookup implementation")
> > Cc: stable@vger.kernel.org
> 
> Is this really stable material? Skipping vzeroupper might have a
> performance impact, but it's not an issue for correctness.
> 
> The main reason why I'm asking is that, while vzeroupper might look
> harmless and obviously safe, it actually caused CVE-2023-20593
> ("ZenBleed") on AMD Zen 2.
> 
> I expect systems receiving stable kernel updates to also run the
> patched microcode by now, so I'm not overly concerned in any case.

It's awkward to mark something as a fix, then not actually want it to be
fixed everywhere.  The stable maintainers know this, and they often
apply fixes anyway regardless of 'Cc stable'.

And if vzeroupper is really not safe due to ZenBleed, then why is it
used everywhere else in the kernel?  This is just one of the few
exceptions that doesn't have it.  If it's not safe then they should all
be alternatives that patch them out to no-ops on affected CPUs.

But since that was never implemented, and userspace almost always uses
vzeroupper too, it seems the only real solution is the microcode.  I
don't think it makes sense to have a middle ground where almost all of
userspace uses vzeroupper, ~97% of the kernel uses vzeroupper, and a
random 3% of the kernel doesn't use it.

- Eric

Re: [PATCH 0/6] x86: add missing vzeroupper instructions

[David Laight]

On Sat, 15 Aug 2026 13:57:44 -0700
Eric Biggers <ebiggers@kernel.org> wrote:

> Assembly code using YMM or ZMM registers is supposed to end with the
> vzeroupper instruction in order to avoid degrading the performance of
> any later SSE code.  Since this only affects performance and not
> correctness, it is sometimes overlooked.  Most kernel code does it
> correctly, but a few cases were missed.  This series fixes them.
> 
> It should be easiest to take the full series through the x86 tree.

Would it be better to an an unconditional vzeroupper in kernel_fpu_end()?
It could go in kernel_fpu_start() but that might have a bigger effect
on latency.
(I assume there is one in kernel_fpu_start() if it actually saves
the user registers?)

Looking the latency/uops seems reasonably on everything 'recent' except
zen-1 and knights-landing.
Although the microcode patch for zen-2 might make that a lot worse.

	David  


> 
> Eric Biggers (6):
>   xor: add missing vzeroupper to AVX code
>   raid6: add missing vzeroupper to AVX2 code
>   raid6: add missing vzeroupper to AVX-512 code
>   crypto: x86/aria - add missing vzeroupper in AVX2 code
>   crypto: x86/aria - add missing vzeroupper in AVX-512 code
>   netfilter: nft_set_pipapo_avx2: add missing vzeroupper
> 
>  arch/x86/crypto/aria-aesni-avx2-asm_64.S  |  6 ++++++
>  arch/x86/crypto/aria-gfni-avx512-asm_64.S |  3 +++
>  lib/raid/raid6/x86/avx2.c                 |  6 ++++++
>  lib/raid/raid6/x86/avx512.c               |  6 ++++++
>  lib/raid/raid6/x86/recov_avx2.c           |  2 ++
>  lib/raid/raid6/x86/recov_avx512.c         |  2 ++
>  lib/raid/xor/x86/xor-avx.c                |  1 +
>  net/netfilter/nft_set_pipapo_avx2.c       | 17 ++++++++---------
>  8 files changed, 34 insertions(+), 9 deletions(-)
> 
> 
> base-commit: db2ddb87143519e20a95aa36c60b36107b736a58

Re: [PATCH 0/6] x86: add missing vzeroupper instructions

[Eric Biggers]

On Sun, Aug 16, 2026 at 04:14:50PM +0100, David Laight wrote:
> On Sat, 15 Aug 2026 13:57:44 -0700
> Eric Biggers <ebiggers@kernel.org> wrote:
> 
> > Assembly code using YMM or ZMM registers is supposed to end with the
> > vzeroupper instruction in order to avoid degrading the performance of
> > any later SSE code.  Since this only affects performance and not
> > correctness, it is sometimes overlooked.  Most kernel code does it
> > correctly, but a few cases were missed.  This series fixes them.
> > 
> > It should be easiest to take the full series through the x86 tree.
> 
> Would it be better to an an unconditional vzeroupper in kernel_fpu_end()?
> It could go in kernel_fpu_start() but that might have a bigger effect
> on latency.
> (I assume there is one in kernel_fpu_start() if it actually saves
> the user registers?)
> 
> Looking the latency/uops seems reasonably on everything 'recent' except
> zen-1 and knights-landing.
> Although the microcode patch for zen-2 might make that a lot worse.
> 
> 	David  

I would like to do that, but there are some issues:

- In some cases, within a single kernel-mode FPU section the kernel
  executes AVX instructions, then SSE instructions afterwards.
  It typically occurs when large input lengths are optimized specially
  with AVX and then shorter lengths fall back to SSE code.
  chacha_dosimd() in lib/crypto/x86/chacha.h is an example of this.

  In these cases the internal vzeroupper is definitely needed.

- Unnecessary overhead if only SSE instructions are used, which is still
  frequent since we don't provide both SSE and AVX versions of the same
  code when the AVX doesn't provide a notable performance benefit.
  lib/crypto/x86/sha256-ni-asm.S is an example of this.

- Further divergence from the userspace ABI, which can be annoying when
  importing assembly code to or from userspace projects, or developing
  or testing the assembly files in userspace.

As for kernel_fpu_begin(), no, it doesn't do vzeroupper.

I do think that some years down the line, we'll drop the use of SSE in
the kernel entirely.  At that point, vzeroupper in kernel_fpu_end()
would make sense.

But until then, I think we should stick with the existing, standard
convention of having the vzeroupper at the end of the assembly routines.

- Eric