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

[Julien Grall]

On 15/03/2019 10:06, Dave Martin wrote:
> 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).

Whoops, I forgot to divide by 8. Thank you for spotting it!

Cheers,

--
Julien Grall