Re: [PATCH] arm64: Call debug_traps_init() from trap_init() to help early kgdb

[Daniel Thompson <daniel.thompson@linaro.org>]

On Fri, May 15, 2020 at 05:23:17PM +0100, Will Deacon wrote:
> On Wed, May 13, 2020 at 04:06:37PM -0700, Douglas Anderson wrote:
> > A new kgdb feature will soon land (kgdb_earlycon) that lets us run
> > kgdb much earlier.  In order for everything to work properly it's
> > important that the break hook is setup by the time we process
> > "kgdbwait".
> > 
> > Right now the break hook is setup in debug_traps_init() and that's
> > called from arch_initcall().  That's a bit too late since
> > kgdb_earlycon really needs things to be setup by the time the system
> > calls dbg_late_init().
> > 
> > We could fix this by adding call_break_hook() into early_brk64() and
> > that works fine.  However, it's a little ugly.  Instead, let's just
> > add a call to debug_traps_init() straight from trap_init().  There's
> > already a documented dependency between trap_init() and
> > debug_traps_init() and this makes the dependency more obvious rather
> > than just relying on a comment.
> > 
> > NOTE: this solution isn't early enough to let us select the
> > "ARCH_HAS_EARLY_DEBUG" KConfig option that is introduced by the
> > kgdb_earlycon patch series.  That would only be set if we could do
> > breakpoints when early params are parsed.  This patch only enables
> > "late early" breakpoints, AKA breakpoints when dbg_late_init() is
> > called.  It's expected that this should be fine for most people.
> > 
> > It should also be noted that if you crash you can still end up in kgdb
> > earlier than debug_traps_init().  Since you don't need breakpoints to
> > debug a crash that's fine.
> > 
> > Suggested-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> > This replaces the patch ("arm64: Add call_break_hook() to
> > early_brk64() for early kgdb") in my recent kgdb series [1].  If I end
> > up re-posting that series again I'll include this patch as a
> > replacement, but I'm sending it separately to avoid spamming a pile of
> > people another time with a 12-patch series.
> > 
> > Note that, because it doesn't select the "ARCH_HAS_EARLY_DEBUG"
> > KConfig option it could be landed standalone.  However, it's still
> > probably better to land together with that patch series.
> > 
> > If the kgdb_earlycon patch series lands without this patch then
> > kgdbwait + kgdb_earlycon won't work well on arm64, but there would be
> > no other bad side effects.
> > 
> > If this patch lands without the kgdb_earlycon patch series then there
> > will be no known problems.
> > 
> > [1] https://lore.kernel.org/r/20200507130644.v4.5.I22067ad43e77ddfd4b64c2d49030628480f9e8d9@changeid
> > 
> >  arch/arm64/include/asm/debug-monitors.h | 2 ++
> >  arch/arm64/kernel/debug-monitors.c      | 4 +---
> >  arch/arm64/kernel/traps.c               | 2 +-
> >  3 files changed, 4 insertions(+), 4 deletions(-)
> 
> [...]
> 
> Acked-by: Will Deacon <will@kernel.org>
> 
> I would prefer to take this via arm64, if possible, since we have quite lot
> going in for 5.8, although I don't think this conflicts at the moment.
> 
> Daniel -- what do you want to do?

I'm very happy for you to take it!

On my side I hope to get the rest of the patchset into linux-next early
next week.


Daniel.