From: legrosbuffle via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>
To: ffmpeg-devel@ffmpeg.org
Cc: legrosbuffle <code@ffmpeg.org>
Subject: [FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss (PR #20446)
Message-ID: <175708684389.25.2130016717995355706@463a07221176> (raw)
PR #20446 opened by legrosbuffle
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20446
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20446.patch
Intel provided a microcode update to mitigate this security
vulnerability which has a huge negative performance impact on gather
instructions. This means that hscale 8to15 avx2, which uses gather
extensively, is no longer faster than SSSE3 on impacted CPUs.
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html
Broadwell:
hscale_8_to_15__fs_4_dstW_512_c: 3379.5 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 615.7 ( 5.49x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 613.4 ( 5.51x)
hscale_8_to_15__fs_4_dstW_512_avx2: 495.7 ( 6.82x)
Skylake:
hscale_8_to_15__fs_4_dstW_512_c: 3411.4 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 591.0 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 591.5 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_avx2: 1386.2 ( 2.46x)
Cascade Lake:
hscale_8_to_15__fs_4_dstW_512_c: 3231.3 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 517.9 ( 6.24x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 521.6 ( 6.19x)
hscale_8_to_15__fs_4_dstW_512_avx2: 1775.0 ( 1.82x)
Sapphire Rapids:
hscale_8_to_15__fs_4_dstW_512_c: 1840.0 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 287.9 ( 6.39x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 293.8 ( 6.26x)
hscale_8_to_15__fs_4_dstW_512_avx2: 219.2 ( 8.40x)
>From 225382486832df97b74f84666aa5f895df503539 Mon Sep 17 00:00:00 2001
From: Alan Kelly <alankelly@google.com>
Date: Fri, 5 Sep 2025 15:17:25 +0000
Subject: [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due
to Intel Gather Data Sampling mitigation performance loss
Intel provided a microcode update to mitigate this security
vulnerability which has a huge negative performance impact on gather
instructions. This means that hscale 8to15 avx2, which uses gather
extensively, is no longer faster than SSSE3 on impacted CPUs.
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html
Broadwell:
hscale_8_to_15__fs_4_dstW_512_c: 3379.5 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 615.7 ( 5.49x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 613.4 ( 5.51x)
hscale_8_to_15__fs_4_dstW_512_avx2: 495.7 ( 6.82x)
Skylake:
hscale_8_to_15__fs_4_dstW_512_c: 3411.4 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 591.0 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 591.5 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_avx2: 1386.2 ( 2.46x)
Cascade Lake:
hscale_8_to_15__fs_4_dstW_512_c: 3231.3 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 517.9 ( 6.24x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 521.6 ( 6.19x)
hscale_8_to_15__fs_4_dstW_512_avx2: 1775.0 ( 1.82x)
Sapphire Rapids:
hscale_8_to_15__fs_4_dstW_512_c: 1840.0 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2: 287.9 ( 6.39x)
hscale_8_to_15__fs_4_dstW_512_ssse3: 293.8 ( 6.26x)
hscale_8_to_15__fs_4_dstW_512_avx2: 219.2 ( 8.40x)
---
libavutil/x86/cpu.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/libavutil/x86/cpu.c b/libavutil/x86/cpu.c
index d6cd4fab9c..1a592f3bf4 100644
--- a/libavutil/x86/cpu.c
+++ b/libavutil/x86/cpu.c
@@ -244,8 +244,9 @@ int ff_get_cpu_flags_x86(void)
family == 6 && model < 23)
rval |= AV_CPU_FLAG_SSSE3SLOW;
- /* Haswell has slow gather */
- if ((rval & AV_CPU_FLAG_AVX2) && family == 6 && model < 70)
+ /* Ice Lake and below have slow gather due to Gather Data Sampling
+ * mitigation. */
+ if ((rval & AV_CPU_FLAG_AVX2) && family == 6 && model < 143)
rval |= AV_CPU_FLAG_SLOW_GATHER;
}
--
2.49.1
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