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

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Date:   Fri, 19 Oct 2018 09:44:18 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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 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.  You could look up the task's PKRU at use_mm time, except that
the task might have disappeared...  In the end just using PKRU=0 makes
some sense and it only should be documented that some kernel services
will ignore PKRU.

Paolo