Subject: Re: sve_user_discard

From: Catalin Marinas <[email-masked]>
Date: Fri, 21 May 2021 10:12:54 +0100

On Thu, May 20, 2021 at 04:02:03PM -0700, [email-masked] wrote:
> This is regarding sve_user_disable(CPACR_EL1_ZEN_EL0EN) on every system
> call.  If a userspace task is using SVE instructions and making sys calls in
> between, it would impact the performance of the thread. On every SVE
> instructions after SVC/system call, it would trap to EL1.
> 
> I think by setting CPACR_EL1_ZEN_EL0EN flag,  the processor faults when it
> runs an SVE instruction. This approach may be taken as part of FPSIMD
> registers switching optimizations.  Can below portion of the code use
> thread.fpsimd_cpu and fpsimd_last_state variables to avoid clearing
> CPACR_EL1_ZEN_EL0EN for this kind of use cases?

There were attempts over the past couple of years to optimise the
syscall return use-case. I think the latest is this one:

https://lore.kernel.org/r/[email-masked]

I'll let Mark comment on his plans for reviving the series. Do you
happen to have some realistic workload that would be improved by this?
We can always write a micro-benchmark but I wonder how much this matters
in the real world.

-- 
Catalin

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