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Date: Fri, 16 Jul 2010 21:30:53 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [patch 2/2] x86 NMI-safe INT3 and Page Fault
Message-ID: <20100716193053.GD7338@basil.fritz.box>
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On Fri, Jul 16, 2010 at 11:25:19AM -0700, Linus Torvalds wrote:
> On Fri, Jul 16, 2010 at 11:15 AM, Avi Kivity <avi@redhat.com> wrote:
> >
> > I think the concern here is about an NMI handler's code running in vmalloc
> > space, or is it something else?
> 
> I think the concern was also potentially doing things like backtraces
> etc that may need access to the module data structures (I think the
> ELF headers end up all being in vmalloc space too, for example).
> 
> The whole debugging thing is also an issue. Now, I obviously am not a
> big fan of remote debuggers, but everybody tells me I'm wrong. And
> putting a breakpoint on NMI is certainly not insane if you are doing
> debugging in the first place. So it's not necessarily always about the
> page faults.

We already have infrastructure for kprobes to prevent breakpoints
on critical code (the __kprobes section). In principle kgdb/kdb
could be taught about honoring those too.

That wouldn't help for truly external JTAG debuggers, but I would assume
those generally can (should) handle any contexts anyways.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.