linux-kernel - Re: [PATCH v3 04/11] kgdb: Delay "kgdbwait" to dbg_late

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Message-ID: <20200430154927.vhkhoffqwirb2fmm@holly.lan>
Date:   Thu, 30 Apr 2020 16:49:27 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     jason.wessel@...driver.com, gregkh@...uxfoundation.org,
        agross@...nel.org, kgdb-bugreport@...ts.sourceforge.net,
        catalin.marinas@....com, linux-serial@...r.kernel.org,
        sumit.garg@...aro.org, corbet@....net, mingo@...hat.com,
        will@...nel.org, hpa@...or.com, tglx@...utronix.de,
        frowand.list@...il.com, bp@...en8.de, bjorn.andersson@...aro.org,
        jslaby@...e.com, Andrew Morton <akpm@...ux-foundation.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 04/11] kgdb: Delay "kgdbwait" to dbg_late_init() by
 default

On Tue, Apr 28, 2020 at 02:13:44PM -0700, Douglas Anderson wrote:
> Using kgdb requires at least some level of architecture-level
> initialization.  If nothing else, it relies on the architecture to
> pass breakpoints / crashes onto kgdb.
> 
> On some architectures this all works super early, specifically it
> starts working at some point in time before Linux parses
> early_params's.  On other architectures it doesn't.  A survey of a few
> platforms:
> 
> a) x86: Presumably it all works early since "ekgdboc" is documented to
>    work here.
> b) arm64: Catching crashes works; with a simple patch breakpoints can
>    also be made to work.
> c) arm: Nothing in kgdb works until
>    paging_init() -> devicemaps_init() -> early_trap_init()
> 
> Let's be conservative and, by default, process "kgdbwait" (which tells
> the kernel to drop into the debugger ASAP at boot) a bit later at
> dbg_late_init() time.  If an architecture has tested it and wants to
> re-enable super early debugging, they can select the
> ARCH_HAS_EARLY_DEBUG KConfig option.  We'll do this for x86 to start.
> It should be noted that dbg_late_init() is still called quite early in
> the system.
> 
> Note that this patch doesn't affect when kgdb runs its init.  If kgdb
> is set to initialize early it will still initialize when parsing
> early_param's.  This patch _only_ inhibits the initial breakpoint from
> "kgdbwait".  This means:
> 
> * Without any extra patches arm64 platforms will at least catch
>   crashes after kgdb inits.
> * arm platforms will catch crashes (and could handle a hardcoded
>   kgdb_breakpoint()) any time after early_trap_init() runs, even
>   before dbg_late_init().
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

It looks like this patch is triggering some warnings from the existing
defconfigs (both x86 and arm64). It looks like this:

---
wychelm$ make defconfig
  GEN     Makefile
*** Default configuration is based on 'x86_64_defconfig'

WARNING: unmet direct dependencies detected for ARCH_HAS_EARLY_DEBUG
  Depends on [n]: KGDB [=n]
  Selected by [y]:
  - X86 [=y]

WARNING: unmet direct dependencies detected for ARCH_HAS_EARLY_DEBUG
  Depends on [n]: KGDB [=n]
  Selected by [y]:
  - X86 [=y]
#
# No change to .config
#
---


Daniel.