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Re: Re: watch exception only for kseg0 addresses..?

To: atul srivastava <atulsrivastava9@rediffmail.com>
Subject: Re: Re: watch exception only for kseg0 addresses..?
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 25 Nov 2002 11:41:09 +0100
Cc: linux-mips@linux-mips.org
In-reply-to: <20021125100152.6471.qmail@mailweb33.rediffmail.com>; from atulsrivastava9@rediffmail.com on Mon, Nov 25, 2002 at 10:01:52AM -0000
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On Mon, Nov 25, 2002 at 10:01:52AM -0000, atul srivastava 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

(It's horrible what mailprograms accounts do to mail formatting ...)

> my cpu manual ( IDT RC32334) talks about two watch registers 
> CP0_IWATCH and CP0_DWATCH where it is required to just put desired 
> VIRTUAL( bits 2--31) addresses to be watched , there is no mention 
> of CP0_WATCHLO and CP0_WATCHHI .
> 
> additionally i guees for userspace virtual watch register problem, 
> the hardware takes care of all , i just need to specify my virual 
> address this is what i understand from my  manual.
> 
> and one more problem i face when i try to debug a mysterious page 
> fault problem, that i get my watch exception but after page fault 
> ..hence I can't really debug , shouldn't the priority of watch 
> exceptions should be higher than atleast instruction fetch 
> exception.? or the scope of debugging by watch exception is 
> limited by design.....

No, the watch exception is one of the lowest priority exceptions.  In case
EXL/ERL are set it might even be defered making it the lowest priority
exception.

  Ralf

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