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

[Will Deacon]

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@xxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>

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?

Will