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

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Date:   Wed, 19 Sep 2018 19:00:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Rik van Riel <riel@...riel.com>, 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 19/09/2018 18:57, Sebastian Andrzej Siewior wrote:
> On 2018-09-19 07:55:51 [+0200], Paolo Bonzini wrote:
>> A kthread can do use_mm/unuse_mm.
> 
> indeed. The FPU struct for the kernel thread isn't valid / does not
> contain the expected PKRU value. So loading the pkru value from the
> struct FPU does not work as expected. We could set it to 0 for a kernel
> thread so we don't end up with a random value.
> If we want to get this usecase working then we would have to move pkru
> value from FPU to mm_struct and consider it in use_mm(). Do we want
> this?

As a start, I think keeping it in the FPU struct but loading it
unconditionally will work.  kthreads will not obey PKU but it will be
better already.

I honestly don't know if PKRU should be per-mm, I don't know mm very
well despite my brilliant observation above. :)

Paolo