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

[Marc Zyngier]

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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > ---
> >
> > 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).

        M.
--
Jazz is not dead. It just smells funny...