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

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Message-Id: <9BAD5065-AA1B-4DEF-A9D4-D7FD9B46667E@amacapital.net>
Date:   Fri, 19 Oct 2018 09:59:00 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        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 Oct 19, 2018, at 12:44 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 18/10/2018 22:46, Andy Lutomirski wrote:
>>> [0] drivers/usb/gadget/function/f_fs.c::ffs_user_copy_worker()
>>> 
>>> Sebastian
>> I think we need an entirely new API:
>> 
>> user_mm_ctx_t ctx = user_mm_ctx_get();
>> 
>> ...
>> 
>> use_user_mm_ctx(ctx);
>> unuse_user_mm_ctx(ctx);
>> 
>> ...
>> 
>> user_mm_ctx_put(ctx);
>> 
>> and ctx will store a copy of mm and PKRU.
>> 
> 
> 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.