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

On Mon, Aug 07, 2023 at 11:47:04AM +0100, Marc Zyngier wrote:

On 2023-08-07 11:28, Mark Rutland wrote:

quoted

On Thu, Jun 01, 2023 at 02:31:51PM -0700, Douglas Anderson wrote:

quoted

From: Sumit Garg <redacted>

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 <redacted>
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.

Thanks,
Mark.

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