linux-kernel - Re: [patch 0/8] x86/fpu: Mop up XSAVES and related damage

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Date:   Fri, 4 Jun 2021 09:27:22 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.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>
Subject: Re: [patch 0/8] x86/fpu: Mop up XSAVES and related damage

On 6/4/2021 7:05 AM, Thomas Gleixner wrote:
> On Wed, Jun 02 2021 at 14:28, Yu-cheng Yu wrote:
>> On 6/2/2021 2:55 AM, Thomas Gleixner wrote:
>>
>> With the series applied, glibc pkey test fails sometimes.  I will try to
>> find out the cause.
> 
> That fails not sometimes. It fails always due to patch 7/8. The reason
> is that before that patch fpu__clear_all() reinitialized the hardware
> which includes writing the initial PKRU value.
> 
> But fpu__initialize() does not store the initial PKRU value in the
> task's xstate which breaks PKRU. As that patch is not urgent we can
> postpone it for not.
> 
> But looking at the above, it's not clear to me why that PKRU stuff works
> at all (upstream), but I'll figure it out eventually. I'm quite sure
> that it does work by pure chance not by design.
>

Thanks for the update.  I will continue looking at it.

Yu-cheng