Messages in this thread | | | Date | Mon, 7 Aug 2023 12:13:56 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI |
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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.
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