linux-kernel - Re: [patch 11/41] x86/fpu: Get rid of copy_supervisor_to

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Date:   Fri, 11 Jun 2021 12:42:22 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch 11/41] x86/fpu: Get rid of copy_supervisor_to_kernel()

On 6/11/21 9:15 AM, 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.

Hallelujah.

Reviewed-by: Andy Lutomirski <luto@...nel.org>