linux-kernel - Re: [RFC PATCH 0/5] running kernel mode SIMD with softirqs disabled

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Message-ID: <YCuZwWWGtqf8PaAf@hirez.programming.kicks-ass.net>
Date:   Tue, 16 Feb 2021 11:09:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-crypto@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <dave.martin@....com>,
        Mark Brown <broonie@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Eric Biggers <ebiggers@...nel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 0/5] running kernel mode SIMD with softirqs disabled

On Fri, Dec 18, 2020 at 06:01:01PM +0100, Ard Biesheuvel wrote:
> [ TL;DR for the non-ARM folks on CC: disabling softirq processing when using
>   SIMD in kernel mode could reduce complexity and improve performance, but we
>   need to decide whether we can do this, and how much softirq processing
>   latency we can tolerate. If we can find a satisfactory solution for this,
>   we might do the same for x86 and 32-bit ARM as well. ]

> - could we do the same on x86, now that kernel_fpu_begin/end is no longer
>   expensive?

Can't we simply save/restore the relevant register set?

So something like (note amluto was wanting to add a regset argument):

	<task>
	kernel_fpu_begin(MMX)
		<SIRQ>
		kernel_fpu_begin(SSE)
		kernel_fpu_end();
		</SIRQ>
	...
	kernel_fpu_end()

Would have to save the MMX regs on first SIRQ invocation of
kernel_fpu_begin(), and then have softirq context termination </SIRQ>
above, restore it.

I mean, we already do much the same for the first kernel_fpu_begin(),
that has to save the user registers, which will be restore when we go
back to userspace.

So why not do exactly the same for softirq context?