linux-kernel - Re: [patch V2 21/52] x86/fpu: Get rid of copy_supervisor_to

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Date:   Thu, 17 Jun 2021 14:41:48 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch V2 21/52] x86/fpu: Get rid of copy_supervisor_to_kernel()

On Mon, Jun 14, 2021 at 05:44:29PM +0200, Thomas Gleixner wrote:
> If the fast path of restoring the FPU state on sigreturn fails or is not
> taken and the current task's FPU is active then the FPU has to be
> deactivated for the slow path to allow a safe update of the tasks FPU
> memory state.
> 
> With supervisor states enabled, this requires to save the supervisor state
> in the memory state first. Supervisor states require XSAVES so saving only
> the supervisor state requires to reshuffle the memory buffer because XSAVES
> uses the compacted format and therefore stores the supervisor states at the
> beginning of the memory state. That's just an overengineered optimization.
> 
> Get rid of it and save the full state for this case.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Reviewed-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/fpu/xstate.h |    1 
>  arch/x86/kernel/fpu/signal.c      |   13 +++++---
>  arch/x86/kernel/fpu/xstate.c      |   55 --------------------------------------
>  3 files changed, 8 insertions(+), 61 deletions(-)

Simplification? To the FPU stinking pile of turds?

Hell yeah!

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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