linux-kernel - Re: [PATCH v2 23/31] arm64: Debugging support

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Message-ID: <20120821085819.GD28660@mudshark.cambridge.arm.com>
Date:	Tue, 21 Aug 2012 09:58:19 +0100
From:	Will Deacon <will.deacon@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ulrich.weigand@...aro.org" <ulrich.weigand@...aro.org>
Subject: Re: [PATCH v2 23/31] arm64: Debugging support

On Mon, Aug 20, 2012 at 09:10:59PM +0100, Arnd Bergmann wrote:
> On Monday 20 August 2012, Will Deacon wrote:
> > On Mon, Aug 20, 2012 at 10:07:54AM +0100, Will Deacon wrote:
> > > On Fri, Aug 17, 2012 at 08:06:07AM +0100, Arnd Bergmann wrote:
> > > > Sorry for the dumb question, but why do you even need PTRACE_GET_THREAD_AREA
> > > > for 64 bit tasks? I thought the thread pointer is a GPR, or is this just
> > > > for compat tasks?
> > > 
> > > The TLS is stored in a co-processor register which is read-only for
> > > userspace.
> > 
> > I should elaborate: the register is RW for AArch64 tasks, RO for aarch32
> > tasks (although that doesn't affect the need for the ptrace request).
> 
> So can't you just /add/ that register to the GPR regset?

We *could*, but it doesn't feel right to me. The TLS register:

	- Can only be manipulated by msr/mrs instructions
	- Is not part of the PCS
	- Is not accessed explicitly by user applications
	- Can not be written via ptrace

so I think it sticks out like a sore thumb if we bundle it up with the GPRs.

Will
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