linux-kernel - Re: [patch V3 52/66] x86/fpu: Add PKRU storage outside of task XSAVE buffer

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Date:   Tue, 22 Jun 2021 18:55:01 +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 52/66] x86/fpu: Add PKRU storage outside of task XSAVE
 buffer

On Fri, Jun 18, 2021 at 04:19:15PM +0200, Thomas Gleixner wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> PKRU is currently partly XSAVE-managed and partly not.  It has space in the
> task XSAVE buffer and is context-switched by XSAVE/XRSTOR.  However, it is
> switched more eagerly than FPU because there may be a need for PKRU to be
> up-to-date for things like copy_to/from_user() since PKRU affects
> user-permission memory accesses, not just accesses from userspace itself.
> 
> This leaves PKRU in a very odd position.  XSAVE brings very little value to
> the table for how Linux uses PKRU except for signal related XSTATE
> handling.
> 
> Prepare to move PKRU away from being XSAVE-managed.  Allocate space in the
> thread_struct for it and save/restore it in the context-switch path
> separately from the XSAVE-managed features. task->thread_struct.pkru is
> only valid when the task is scheduled out. For the current task the
> authoritative source is the hardware, i.e. it has to be retrieved via
> rdpkru().
> 
> Leave the XSAVE code in place for now to ensure bisectability.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V3: Fix the fallout on !PKRU enabled systems in copy_thread() - Intel testing via Dave
> ---
>  arch/x86/include/asm/processor.h |    9 +++++++++
>  arch/x86/kernel/process.c        |    7 +++++++
>  arch/x86/kernel/process_64.c     |   25 +++++++++++++++++++++++++
>  3 files changed, 41 insertions(+)

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

-- 
Regards/Gruss,
    Boris.

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