linux-kernel - Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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Date:   Thu, 21 Feb 2019 14:06:26 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Will Deacon <will.deacon@....com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch




> On Feb 21, 2019, at 4:46 AM, Will Deacon <will.deacon@....com> wrote:
> 
>> On Wed, Feb 20, 2019 at 02:55:59PM -0800, H. Peter Anvin wrote:
>>> On 2/19/19 4:48 AM, Will Deacon wrote:
>>> 
>>> I think you'll still hate this, but could we not disable preemption during
>>> the uaccess-enabled region, re-enabling it on the fault path after we've
>>> toggled uaccess off and disable it again when we return back to the
>>> uaccess-enabled region? Doesn't help with tracing, but it should at least
>>> handle the common case.
>>> 
>> 
>> There is a worse problem with this, I still realize: this would mean blocking
>> preemption across what could possibly be a *very* large copy_from_user(), for
>> example.
> 
> I don't think it's legitimate to call copy_{to,from}_user() inside a
> user_access_{begin,end} region. You'd need to add some unsafe variants,
> which could periodically disable uaccess and call cond_resched() inside
> the loop to avoid the problem you're eluding to.
> 

Definitely not safe. On x86 they do CLAC and everything breaks.