linux-kernel - Re: Dealing with the NMI mess

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Date:	Thu, 23 Jul 2015 14:54:54 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
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 2:45 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> 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.

I'd really prefer it if we don't touch NMI code in those kinds of
ways. The NMI code is fragile as hell. All the problems we have with
it is exactly due to "where is the boundary" issues.

That's why I *don't* want NMI code to do magic crap. Anything that
says "disable this during this magic window" is broken. The problems
we've had are exactly about atomicity of the entry/exit conditions,
and there is no really good way to get them right.

I'd be much happier with a _TIF_USER_WORK_MASK approach exactly
because it's so *obvious* that it's not a boundary condition.

I dislike the "disable and re-enable dr7 in the NMI handler" exactly
because it smells like "we can only handle faults in _this_ region".
It may be true, but it's also what I want us to get away from. I'd
much rather have the "big picture" be that we can take faults anywhere
at all (*), and that none of the core code really cares. Then we "fix
up" user space.

                   Linus

(*) And yes, sysenter and not having a stack at all is very special,
and I think we will *always* have to have that magical special case of
the first few instructions there. But that's a separate hardware
limitation we can't get around.
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