linux-kernel - Re: [PATCH v9 00/27] x86: load FPU registers on return to userland

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Message-ID: <alpine.DEB.2.21.1904081907280.2352@nanos.tec.linutronix.de>
Date:   Mon, 8 Apr 2019 19:08:49 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        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 v9 00/27] x86: load FPU registers on return to userland

On Wed, 3 Apr 2019, Sebastian Andrzej Siewior wrote:

> 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.

So aside of that one hunk in 24/27 which is either wrong or needs some
information in the changelog I couldn't find anything disturbing.

With that addressed:

     Reviewed-by: Thomas Gleixner <tglx@...utronix.de>