Re: [PATCH 2/2] meson: Set avx512f option to auto

[Richard Henderson <richard.henderson@linaro.org>]

On 12/16/22 15:08, Paolo Bonzini wrote:
> Because that's what configure used to do 
> (https://lists.nongnu.org/archive/html/qemu-devel/2022-02/msg00650.html). 
> <https://lists.nongnu.org/archive/html/qemu-devel/2022-02/msg00650.html).>..

Yeah, but I wondered if that was just a bug.

> It can surely be changed but AVX512 is known to limit processor frequency. I am not sure 
> if the limitation is per core or extends to multiple cores, and it would be a pity if 
> guests were slowed down even further during migration.

Hmm.  Should we simply remove it?

> Especially after the bulk phase buffer_is_zero performance matters a lot less so you'd pay 
> the price of AVX512 for little gain. After the bulk phase it may even make sense to just 
> use SSE, since even AVX requires a voltage transition[1] from what I saw at 
> https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html 
> <https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html>.

Ouch, never heard of that.

I'm not going to worry about it, because glibc str* routines make the same choice to use 
AVX2, as does TCG, so I can only imagine that for the most part we're continually in and 
out of 256-bit avx mode.

Anyway, I'll drop this patch.


r~