linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CA+55aFxMXg=txUPSGCHd__neQq=EU0jU051kKr0GmYBZ=iLWSg@mail.gmail.com>
Date:	Thu, 23 Jul 2015 14:35:08 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andy Lutomirski <luto@...capital.net>, 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 2:20 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.

               Linus
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