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

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Date:   Tue, 18 Sep 2018 11:11:20 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Paolo Bonzini <pbonzini@...hat.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 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.

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