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

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Message-ID: <20160510104511.GH687@leverpostej>
Date:	Tue, 10 May 2016 11:45:11 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....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>,
	Vijaya Kumar K <Vijaya.Kumar@...iumnetworks.com>
Subject: Re: [PATCH] arm64: kgdb: Match pstate size with gdbserver protocol

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.

While that was how we discovered the inconsistency, a major concern is
that SPSR_EL* (i.e. PSTATE), as accessed by MRS/MSR is a 64-bit
quantity, even if the upper 32 bits are RES0 today.

It is conceivable that the upper 32 bits could be used in future (as
happened with CLIDR_EL1), and for this reason we expose those upper 32
bits from the kernel, and treat system registers as 64-bit quantities
generally.

So this was also about ensuring the interface was consistent and to some
extent future-proof.

Thanks,
Mark.