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

To: "Ralf Baechle" <ralf@linux-mips.org>
Subject: Re: Re: watch exception only for kseg0 addresses..?
From: "atul srivastava" <atulsrivastava9@rediffmail.com>
Date: 25 Nov 2002 10:01:52 -0000
Cc: linux-mips@linux-mips.org
Original-recipient: rfc822;linux-mips@linux-mips.org
Reply-to: "atul srivastava" <atulsrivastava9@rediffmail.com>
Sender: linux-mips-bounce@linux-mips.org

> in change history of this file i am able to see KSEG0 restriction
> removed only for arch/mips64/lib/watch.S...

The hw takes physical addresses, so using a a virtual address as argument >for __watch_set seemed to be stupid anyway. The hw takes a physical address and the conversion is best done in C anyway.

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

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.....





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