linux-kernel - Re: [RFC PATCH 1/2] hw-breakpoints: Separate constraint space for data and instruction breakpoints

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Message-ID: <1271428530.23489.3.camel@e102144-lin.cambridge.arm.com>
Date:	Fri, 16 Apr 2010 15:35:30 +0100
From:	Will Deacon <will.deacon@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
	"K . Prasad" <prasad@...ux.vnet.ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH 1/2] hw-breakpoints: Separate constraint space for
 data and instruction breakpoints

Hello Frederic,

Thanks for this.

On Tue, 2010-04-13 at 00:01 +0100, Frederic Weisbecker wrote:
> There are two outstanding fashions for archs to implement hardware
> breakpoints.
> 
> The first is to separate breakpoint address pattern definition
> space between data and instruction breakpoints. We then have
> typically distinct instruction address breakpoint registers
> and data address breakpoint registers, delivered with
> separate control registers for data and instruction breakpoints
> as well. This is the case of PowerPc and ARM for example.
> 
> The second consists in having merged breakpoint address space
> definition between data and instruction breakpoint. Address
> registers can host either instruction or data address and
> the access mode for the breakpoint is defined in a control
> register. This is the case of x86.
> 
> This patch adds a new CONFIG_HAVE_MIXED_BREAKPOINTS_REGS config
> that archs can select if they belong to the second case. Those
> will have their slot allocation merged for instructions and
> data breakpoints.
> 
> The others will have a separate slot tracking between data and
> instruction breakpoints.

This looks useful for supporting architectures with separate
data/instruction breakpoints.

A couple of points:

1.) Will this affect the arch backend at all?

2.) On ARM, it is possible to have different numbers of breakpoint registers
    and watchpoint registers [which we do not know until runtime]. This
    means that we have to define HBP_NUM as a potential upper bound,
    which seems a bit wasteful. Perhaps there could be a mechanism to 
    register the available resources at runtime?

Thanks,

Will



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