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

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Message-ID: <20200504144306.zwac2jdlltvhekqm@holly.lan>
Date:   Mon, 4 May 2020 15:43:06 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     x86@...nel.org, Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>, bp@...en8.de,
        linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [Kgdb-bugreport] [PATCH v3 04/11] kgdb: Delay "kgdbwait" to
 dbg_late_init() by default

On Thu, Apr 30, 2020 at 09:35:30AM -0700, Doug Anderson wrote:
> 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.

Happy with the approach to fix this.

Given the follow on discussion from the end of last week I suspect there
probably needs to be a v4 anyway so perhaps the last question is
applying a fix up is moot at this point?


Daniel.