linux-kernel - Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI

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Message-ID: <a030019c7c5887d54b97054ed7d9af12@kernel.org>
Date:   Mon, 07 Aug 2023 11:47:04 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-perf-users@...r.kernel.org, ito-yuichi@...itsu.com,
        Chen-Yu Tsai <wens@...e.org>, Ard Biesheuvel <ardb@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        kgdb-bugreport@...ts.sourceforge.net,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Wei Li <liwei391@...wei.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 7/7] arm64: kgdb: Roundup cpus using the debug IPI

On 2023-08-07 11:28, Mark Rutland wrote:
> On Thu, Jun 01, 2023 at 02:31:51PM -0700, Douglas Anderson wrote:
>> From: Sumit Garg <sumit.garg@...aro.org>
>> 
>> Let's use the debug IPI for rounding up CPUs in kgdb. When the debug
>> IPI is backed by an NMI (or pseudo NMI) then this will let us debug
>> even hard locked CPUs. When the debug IPI isn't backed by an NMI then
>> this won't really have any huge benefit but it will still work.
>> 
>> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
>> Signed-off-by: Douglas Anderson <dianders@...omium.org>
>> ---
>> 
>> Changes in v9:
>> - Remove fallback for when debug IPI isn't available.
>> - Renamed "NMI IPI" to "debug IPI" since it might not be backed by 
>> NMI.
>> 
>>  arch/arm64/kernel/ipi_debug.c |  5 +++++
>>  arch/arm64/kernel/kgdb.c      | 14 ++++++++++++++
>>  2 files changed, 19 insertions(+)
> 
> This looks fine to me, but I'd feel a bit happier if we had separate 
> SGIs for
> the backtrace and the KGDB callback as they're logically unrelated.

Well, we're a bit stuck here.

We have exactly *one* spare SGI with GICv3, as we lose 8 of them
to the secure side. One possibility would be to mux some of the
lesser used IPIs onto two SGIs (one with standard priority, and
one with NMI priority).

         M.
-- 
Jazz is not dead. It just smells funny...