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Re: watch exception only for kseg0 addresses..?

To: Ralf Baechle <>
Subject: Re: watch exception only for kseg0 addresses..?
From: "Maciej W. Rozycki" <>
Date: Mon, 25 Nov 2002 12:55:11 +0100 (MET)
Cc: atul srivastava <>,
In-reply-to: <>
Organization: Technical University of Gdansk
Original-recipient: rfc822;
On Mon, 25 Nov 2002, Ralf Baechle wrote:

> The whole watch stuff in the the kernel is pretty much an ad-hoc API
> which I did create to debug a stack overflow.  I'm sure if you're
> going to use it you'll find problems.  For userspace for example you'd
> have to switch the watch register when switching the MMU context so
> each process gets it's own virtual watch register.  Beyond that there
> are at least two different formats of watch registers implemented in
> actual silicon, the original R4000-style and the MIPS32/MIPS64 style
> watch registers and the kernel's watch code only know the R4000 style
> one.  So check your CPU's manual ...

 I think the best use of the watch exception would be making it available
to userland via PTRACE_PEEKUSR and PTRACE_POKEUSR for hardware watchpoint
support (e.g. for gdb).  Hardware support is absolutely necessary for
watching read accesses and much beneficial for write ones (otherwise gdb
single-steps code which sucks performace-wise).

+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

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