linux-kernel - Re: Dealing with the NMI mess

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Message-ID: <CA+55aFzma9NgODkzz08zpEKSWVnwxuCvwPt_JnO8HaHwRnBPdQ@mail.gmail.com>
Date:	Thu, 23 Jul 2015 14:59:56 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Steven Rostedt <rostedt@...dmis.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>,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Thu, Jul 23, 2015 at 2:50 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> What if we relax it slightly: "if the breakpoint happened during that
> interrupts-off region, I will clear all *kernel breakpoints* in %dr7
> to guarantee forward progress"?
>
> Watchpoints don't need RF to make forward progress, and, by leaving
> watchpoints alone, we avoid breaking gdb.

Hmmm. I thought watchpoints were "before the instruction" too, but
that's just because I haven't used them in ages, and I didn't remember
the details. I just looked it up.

You're right - the memory watchpoints trigger after the instruction
has executed, so RF isn't an issue. So yes, the only issue is
instruction breakpoints, and those are the only ones we need to clear.

And that makes it really easy.

So yes, I agree. We only need to clear all kernel breakpoints.

So we don't even need that _TIF_USER_WORK_MASK thing, because user
space isn't setting kernel code breakpoints, it's just kgdb.

Sounds good to me.

                Linus
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