Re: [FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss - Alan Kelly via ffmpeg-devel

From: Alan Kelly via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>
To: ffmpeg-devel@ffmpeg.org
Cc: Alan Kelly <alankelly@google.com>
Subject: Re: [FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss
Date: Thu, 21 Aug 2025 13:57:09 +0200
Message-ID: <CABaBdZrqow7LXzKU4w_4=sWK1f3Qebbsv9VZaCBu2nR-hut07w@mail.gmail.com> (raw)
In-Reply-To: <CABaBdZoABsGnNgB4i+xSnK1c1k6mT5b=1_8nSbfp6GDHrsPENQ@mail.gmail.com>

On Fri, Aug 8, 2025 at 2:23 PM Alan Kelly <alankelly@google.com> wrote:

>
>
> On Fri, Aug 8, 2025 at 2:21 PM Alan Kelly <alankelly@google.com> wrote:
>
>> 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.
>> ---
>>  libavutil/x86/cpu.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavutil/x86/cpu.c b/libavutil/x86/cpu.c
>> index d6cd4fab9c..923c63e0c4 100644
>> --- a/libavutil/x86/cpu.c
>> +++ b/libavutil/x86/cpu.c
>> @@ -244,9 +244,11 @@ 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;
>> +        }
>>      }
>>
>>  #endif /* cpuid */
>> --
>> 2.50.1.703.g449372360f-goog
>>
>
>
> 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)
>
>
>

Hi,

Are there any objections to the patch? The performance impact is huge, so
it should be patched quickly.

Thanks,

Alan
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