Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

[Dave Martin]

On Thu, Mar 14, 2019 at 06:07:19PM +0000, Julien Grall wrote:
> Hi Sebastian,
> 
> On 3/4/19 5:25 PM, Sebastian Andrzej Siewior wrote:

[...]

> >It would increase the softirq latency but the question is how bad would
> >it be. It would continue once the SIMD section is done.
> 
> On Arm, the kernel may use either FPSIMD or SVE (if supported by the
> platform). While the FPSIMD context is fairly small (~4K), the SVE context
> can be up to ~64KB.

It's not quite as bad as that.  The FPSIMD state is ~0.5K, with the SVE
state size being up to ~8.5K (though for today's implementations ~2K may
be considered typical).

For comparision, I believe AVX-512 has ~2K of state.

Cheers
---Dave