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

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Message-ID: <823a0f8e-5c08-cd4d-851b-9e8e147fe01b@redhat.com>
Date:   Wed, 12 Sep 2018 17:30:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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] x86/fpu: eager switch PKRU state

On 12/09/2018 17:24, Andy Lutomirski wrote:
>> I think you can go a step further and exclude PKRU state from
>> copy_kernel_to_fpregs altogether; you just use RDPKRU/WRPKRU.  This also
>> means you don't need to call __fpregs_* functions in write_pkru.
>>
>>
> Except that the signal ABI has PKRU in the xstate. So we’d need to fake it or do something special for signals.

The signal ABI is already special because it uses the non-compacted
format.  As long as copy_fpregs_to_sigframe includes the PKRU state
(i.e. EDX:EAX=-1), and PKRU value is okay (which it is because it's
switched eagerly), everything should work...

Paolo