linux-kernel - Re: [PATCH] arm64: kgdb: Match pstate size with gdbserver protocol

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Message-ID: <20160510102838.GF22813@arm.com>
Date:	Tue, 10 May 2016 11:28:38 +0100
From:	Will Deacon <will.deacon@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	Jason Wessel <jason.wessel@...driver.com>,
	Dave P Martin <Dave.Martin@....com>,
	Mark Rutland <mark.rutland@....com>,
	Vijaya Kumar K <Vijaya.Kumar@...iumnetworks.com>
Subject: Re: [PATCH] arm64: kgdb: Match pstate size with gdbserver protocol

Hi Daniel,

So does kgdb do something useful after this patch?

On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote:
> Current versions of gdb do not interoperate cleanly with kgdb on arm64
> systems because gdb and kgdb do not use the same register description.
> This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1).
> 
> Compatibility with gdb (after the patch is applied) is as follows:
> 
>   gdb-7.6 and earlier  Ok
>   gdb-7.7 series       Works if user provides custom target description
>   gdb-7.8(.0)          Works if user provides custom target description
>   gdb-7.8.1 and later  Ok
> 
> When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was
> introduced it was paired with a gdb patch that made an incompatible
> change to the gdbserver protocol. This patch was eventually merged into
> the gdb sources:
> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd
> 
> The change to the protocol was mostly made to simplify big-endian support
> inside the kernel gdb stub. Unfortunately the gdb project released
> gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified
> and reversed:
> https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee
> 
> This leaves us in a position where kgdb still uses the no-longer-used
> protocol; gdb-7.8.1, which restored the original behaviour,If was
> released on 2014-10-29.
> 
> I don't believe it is possible to detect/correct the protocol
> incompatiblity which means the kernel must take a view about which
> version of the gdb remote protocol is "correct". This patch takes the
> view that the original/current version of the protocol is correct
> and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous.

Urgh, this is filthy! Still, without a time machine, I guess there's
little we can do about it. Can I ask you to respin the patch but with
the rationale as a comment in the header file, and a pointer to the
comment from the C code too, please?

The code looks incorrect after this change, so we should justify how
we've ended up in this state and not everybody looks at the git log
for that rationale.

Cheers,

Will