Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Aug 07, 2023 at 12:08:06PM +0100, Marc Zyngier wrote:
> On 2023-08-07 11:54, Mark Rutland wrote:
> > On Mon, Aug 07, 2023 at 11:47:04AM +0100, Marc Zyngier wrote:
> > > On 2023-08-07 11:28, Mark Rutland wrote:
> > > > On Thu, Jun 01, 2023 at 02:31:51PM -0700, Douglas Anderson wrote:
> > > > > From: Sumit Garg <sumit.garg@linaro.org>
> > > > >
> > > > > Let's use the debug IPI for rounding up CPUs in kgdb. When the debug
> > > > > IPI is backed by an NMI (or pseudo NMI) then this will let us debug
> > > > > even hard locked CPUs. When the debug IPI isn't backed by an NMI then
> > > > > this won't really have any huge benefit but it will still work.
> > > > >
> > > > > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > > > > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > > > ---
> > > > >
> > > > > Changes in v9:
> > > > > - Remove fallback for when debug IPI isn't available.
> > > > > - Renamed "NMI IPI" to "debug IPI" since it might not be backed by
> > > > > NMI.
> > > > >
> > > > >  arch/arm64/kernel/ipi_debug.c |  5 +++++
> > > > >  arch/arm64/kernel/kgdb.c      | 14 ++++++++++++++
> > > > >  2 files changed, 19 insertions(+)
> > > >
> > > > This looks fine to me, but I'd feel a bit happier if we had separate
> > > > SGIs for
> > > > the backtrace and the KGDB callback as they're logically unrelated.
> > > 
> > > Well, we're a bit stuck here.
> > > 
> > > We have exactly *one* spare SGI with GICv3, as we lose 8 of them
> > > to the secure side. One possibility would be to mux some of the
> > > lesser used IPIs onto two SGIs (one with standard priority, and
> > > one with NMI priority).
> > 
> > Understood; Doug and I suggested two options for that:
> > 
> > 1) Unify/mux the IPI_CPU_STOP and IPI_CPU_CRASH_STOP IPIs
> > 
> >    The only *intended* difference between the two is that
> > IPI_CPU_CRASH_STOP
> >    calls crash_save_cpu() before trying to stop the CPU, but the
> >    implementations have diverged significantly for unrelated reasons.
> > 
> > 2) Remove IPI_WAKEUP
> > 
> >    We only use IPI_WAKEUP for the ACPI parking protocol, and we could
> > reuse
> >    another IPI (e.g. IPI_RESCHEDULE) to achieve the same thing witout a
> >    dedicated IPI.
> 
> Sure. My concern is that we're papering over the fundamental problem,
> which is that IPIs are limited resource, and that we're bound to pile
> more stuff on them.
> 
> I'm all for reclaiming the ones that can be merged, but we may ultimately
> need a real fix for this.

Sure; I will bear that in mind.

Thanks,
Mark.