Re: [Qemu-devel] [PATCH 2/2 V7] qemu, qmp: add inject-nmi qmp command

[Jamie Lokier <jamie@shareable.org>]

Blue Swirl wrote:
> On Fri, Apr 8, 2011 at 9:04 AM, Gleb Natapov <gleb@redhat.com> wrote:
> > On Thu, Apr 07, 2011 at 04:41:03PM -0500, Anthony Liguori wrote:
> >> On 04/07/2011 02:17 PM, Gleb Natapov wrote:
> >> >On Thu, Apr 07, 2011 at 10:04:00PM +0300, Blue Swirl wrote:
> >> >>On Thu, Apr 7, 2011 at 9:51 PM, Gleb Natapov<gleb@redhat.com>  wrote:
> >> >>
> >> >>I'd prefer something more generic like these:
> >> >>raise /apic@fee00000:l1int
> >> >>lower /i44FX-pcihost/e1000@03.0/pinD
> >> >>
> >> >>The clumsier syntax shouldn't be a problem, since this would be a
> >> >>system developer tool.
> >> >>
> >> >>Some kind of IRQ registration would be needed for this to work without
> >> >>lots of changes.
> >> >True. The ability to trigger any interrupt line is very useful for
> >> >debugging. I often re-implement it during debug.
> >>
> >> And it's a good thing to have, but exposing this as the only API to
> >> do something as simple as generating a guest crash dump is not the
> >> friendliest thing in the world to do to users.
> >>
> > Well, this is not intended to be used by regular users directly and
> > management can provide nicer interface for issuing NMI. But really,
> > my point is that NMI actually generates guest core dump in such rare
> > cases (only preconfigured Windows guests) that it doesn't warrant to
> > name command as such. Management is in much better position to implement
> > functionality with such name since it knows what type of guest it runs
> > and can tell agent to configure guest accordingly.
> 
> Does the management need to know about each and every debugging
> oriented interface? For example, "info regs",  "info mem", "info irq"
> and tracepoints?

Linux uses NMI for performance tracing, profiling, watchdog etc. so in
practice, NMI is very similar to the other IRQs.  I.e. highly guest
specific and depending on what's wired up to it.  Injecting NMI to all
CPUs at once does not make any sense for those Linux guests.

For Windows crash dumps, I think it makes sense to have a "button
wired to NMI" device, rather than inject-nmi directly, but I can see
that inject-nmi solves the intended problem quite neatly.

For Linux crash dumps, for example, there are other key combinations,
as well as watchdog devices, that can be used to trigger them.  A
virtual "button wired to GPIO/PCI-IRQ/etc." device might be quite
handy for debugging Linux guests, and would fit comfortably in a
management interface.

-- Jamie