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

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Message-ID: <7e9a13f3-93f5-fe4a-20d2-f4f9407bd43b@redhat.com>
Date:   Tue, 18 Sep 2018 17:07:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state

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.

Paolo

>> Also, a __read_pkru() seems to be missing in switch_fpu_prepare.
> But the value is stored during write_pkru(). So the copy that is saved
> should be identical to the value that would be read, correct?
>