linux-kernel - Re: [patch V2 45/52] x86/fpu: Dont restore PKRU in fpregs_restore

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Message-ID: <89da2d23-7de8-8dee-5b6a-361b7ceeae8e@intel.com>
Date:   Tue, 15 Jun 2021 17:52:32 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...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 V2 45/52] x86/fpu: Dont restore PKRU in
 fpregs_restore_userspace()

On 6/14/2021 8:44 AM, 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>
> ---
>   arch/x86/include/asm/fpu/internal.h |   12 +++++++++++-
>   arch/x86/include/asm/fpu/xstate.h   |   19 +++++++++++++++++++
>   arch/x86/kernel/fpu/core.c          |    2 +-
>   3 files changed, 31 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -455,7 +455,17 @@ static inline void fpregs_restore_userre
>   		return;
>   
>   	if (!fpregs_state_valid(fpu, cpu)) {
> -		restore_fpregs_from_fpstate(&fpu->state);
> +		/*
> +		 * This restores _all_ xstate which has not been
> +		 * established yet.
> +		 *
> +		 * If PKRU is enabled, then the PKRU value is already
> +		 * correct because it was either set in switch_to() or in
> +		 * flush_thread(). So it is excluded because it might be
> +		 * not up to date in current->thread.fpu.xsave state.
> +		 */
> +		__restore_fpregs_from_fpstate(&fpu->state,
> +					      xfeatures_mask_restore_user());

This needs to be xfeatures_mask_restore_user() | 
xfeatures_mask_supervisor().

>   		fpregs_activate(fpu);
>   		fpu->last_cpu = cpu;
>   	}

[...]