linux-kernel - Re: [patch V3 51/66] x86/fpu: Dont restore PKRU in fpregs_restore

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Message-ID: <YNIK632EfZXPQw5s@zn.tnic>
Date:   Tue, 22 Jun 2021 18:08:11 +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 V3 51/66] x86/fpu: Dont restore PKRU in
 fpregs_restore_userspace()

On Fri, Jun 18, 2021 at 04:19:14PM +0200, Thomas Gleixner wrote:
> switch_to(), flush_thread() write the task's PKRU value eagerly so the PKRU
> value of current is always valid in the hardware.
> 
> That means there is no point in restoring PKRU on exit to user or when
> reactivating the task's FPU registers in the signal frame setup path.
> 
> This allows to remove all the xstate buffer updates with PKRU values once
> the PKRU state is stored in thread struct while a task is scheduled out.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V3: Restore supervisor state too. (Yu-Cheng)
> ---
>  arch/x86/include/asm/fpu/internal.h |   16 +++++++++++++++-
>  arch/x86/include/asm/fpu/xstate.h   |   19 +++++++++++++++++++
>  arch/x86/kernel/fpu/core.c          |    2 +-
>  3 files changed, 35 insertions(+), 2 deletions(-)

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

-- 
Regards/Gruss,
    Boris.

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