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

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Date:   Thu, 30 Apr 2020 09:35:30 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Gross <agross@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Catalin Marinas <catalin.marinas@....com>,
        linux-serial@...r.kernel.org, Sumit Garg <sumit.garg@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frank Rowand <frowand.list@...il.com>, bp@...en8.de,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Jiri Slaby <jslaby@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: [PATCH v3 04/11] kgdb: Delay "kgdbwait" to dbg_late_init() by default

Hi,

On Thu, Apr 30, 2020 at 8:49 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> 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]

Ah, thanks!  I hadn't noticed those.  I think it'd be easy to just
change the relevant patches to just "select ARCH_HAS_EARLY_DEBUG if
KGDB".  If you agree that's a good fix and are willing, I'd be happy
if you just added it to the relevant patches when applying.  If not, I
can post a v4.

-Doug