linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 23 Jul 2015 14:45:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Willy Tarreau <w@....eu>, Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 2:35 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jul 23, 2015 at 2:20 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> So the NMI could trigger userspace debug register faults, and simply
>> disabling them would make the whole debug register thing entirely
>> unreliable.
>
> We could easily set something to re-enable them for when we actually
> return to user space. I'd be ok with just setting the
> _TIF_USER_WORK_MASK.
>
> But even that should not be a requirement for the basic stability and
> core integrity of the kernel. Not like the current horrid mess with
> NMI nesting and ESP fixing etc.
>
> And realistically, nobody will ever even notice. So the whole "ok, we
> can use _TIF_USER_WORK_MASK to re-enable dr7" is a tiny tiny detail
> that is more like cleaning up things, not a core issue.
>

Or we just re-enable them on the way out of NMI (i.e. the very last
thing we do in the NMI handler).  I don't want to break regular
userspace gdb when perf is running.

--Andy
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