linux-kernel - Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state

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Message-ID: <36e8493f-f994-e885-8fe6-2f0d4a9904a1@redhat.com>
Date:   Tue, 18 Sep 2018 17:29:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...riel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state

On 18/09/2018 17:11, Rik van Riel wrote:
> On Tue, 2018-09-18 at 17:07 +0200, Paolo Bonzini wrote:
>> On 18/09/2018 16:27, Sebastian Andrzej Siewior wrote:
>>>> Likewise, move this to fpu__clear and outside "if
>>>> (static_cpu_has(X86_FEATURE_FPU))"?
>>>
>>> okay. But if there is no FPU we did not save/restore the pkru
>>> value. Is
>>> this supposed to be an improvement?
>>
>> Honestly it just seemed "more correct", but now that I think about
>> it,
>> kernel threads should run with PKRU=0.  maybe there's a preexisting
>> bug
>> that your patch has the occasion to fix.
> 
> I don't think it matters what the PKRU state is
> for kernel threads, since kernel PTEs should not
> be using protection keys anyway.

What about copy_from/to_user?

Paolo



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