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SubjectRe: Dealing with the NMI mess
On Thu, Jul 23, 2015 at 2:35 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jul 23, 2015 at 2:20 PM, Peter Zijlstra <peterz@infradead.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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