linux-kernel - RE: [PATCH v6] x86: load FPU registers on return to userland

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Date:   Tue, 15 Jan 2019 12:44:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Andrzej Siewior' <bigeasy@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: RE: [PATCH v6] x86: load FPU registers on return to userland

From:  Sebastian Andrzej Siewior
> Sent: 09 January 2019 11:47
>
> This is a refurbished series originally started by by Rik van Riel. The
> goal is load the FPU registers on return to userland and not on every
> context switch. By this optimisation we can:
> - avoid loading the registers if the task stays in kernel and does
>   not return to userland
> - make kernel_fpu_begin() cheaper: it only saves the registers on the
>   first invocation. The second invocation does not need save them again.
> 
> To access the FPU registers in kernel we need:
> - disable preemption to avoid that the scheduler switches tasks. By
>   doing so it would set TIF_NEED_FPU_LOAD and the FPU registers would be
>   not valid.
> - disable BH because the softirq might use kernel_fpu_begin() and then
>   set TIF_NEED_FPU_LOAD instead loading the FPU registers on completion.

Once this is done it might be worth while adding a parameter to
kernel_fpu_begin() to request the registers only when they don't
need saving.
This would benefit code paths where the gains are reasonable but not massive.

The return value from kernel_fpu_begin() ought to indicate which
registers are available - none, SSE, SSE2, AVX, AVX512 etc.
So code can use an appropriate implementation.
(I've not looked to see if this is already the case!)

	David

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