linux-kernel - Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

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Message-ID: <9c5ed364-4282-7dd3-f614-4e62632d81aa@linux.intel.com>
Date:   Fri, 19 Oct 2018 10:01:07 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Radim Krcmar <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

On 10/19/2018 09:59 AM, Andy Lutomirski wrote:
>> That looks like a good API in general.  The ffs_user_copy_worker that
>> Sebastian mentioned seems to be used by AIO, in which case of course it
>> has to happen in a kernel thread.
>>
>> But while the API is good, deciding on the desired semantics is
>> "interesting".  The submitting thread might be changing PKRU between the
>> time the I/O operation is submitted and the time it is completed, for
>> example.
> I think there’s only one sensible answer: capture PKRU at the time of submission.

I think it's much more straightforward to just not enforce pkeys.
Having this "phantom" value could cause a very odd, nearly undebuggable
I/O failure.