Re: [RFC] [PATCH] powerpc: Add MSR_DE to MSR_KERNEL

Scott Wood [off-list ref] wrote on 2012/06/01 00:16:53:

On 05/31/2012 05:14 PM, Joakim Tjernlund wrote:

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Scott Wood [off-list ref] wrote on 2012/05/31 23:43:34:

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On 05/31/2012 04:38 PM, Joakim Tjernlund wrote:

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Scott Wood [off-list ref] wrote on 2012/05/31 19:47:53:

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On 05/31/2012 04:56 AM, Joakim Tjernlund wrote:

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Abatron Support [off-list ref] wrote on 2012/05/31 11:30:57:

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Abatron Support [off-list ref] wrote on 2012/05/30 14:08:26:

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I have tested this briefly with BDI2000 on P2010(e500) and
it works for me. I don't know if there are any bad side effects,
therfore
this RFC.

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We used to have MSR_DE surrounded by CONFIG_something
to ensure it wasn't set under normal operation.  IIRC, if MSR_DE
is set, you will have problems with software debuggers that
utilize the the debugging registers in the chip itself.  You only want
to force this to be set when using the BDI, not at other times.

This MSR_DE is also of interest and used for software debuggers that
make use of the debug registers. Only if MSR_DE is set then debug
interrupts are generated. If a debug event leads to a debug interrupt
handled by a software debugger or if it leads to a debug halt handled
by a JTAG tool is selected with DBCR0_EDM / DBCR0_IDM.

The "e500 Core Family Reference Manual" chapter "Chapter 8
Debug Support" explains in detail the effect of MSR_DE.

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So what is the verdict on this? I don't buy into Dan argument without some
hard data.

What I tried to mention is that handling the MSR_DE correct is not only
an emulator (JTAG debugger) requirement. Also a software debugger may
depend on a correct handled MSR_DE bit.

Yes, that made sense to me too. How would SW debuggers work if the kernel keeps
turning off MSR_DE first chance it gets?

The kernel selectively enables MSR_DE when it wants to debug.  I'm not
sure if anything will be bothered by leaving it on all the time.  This
is something we need for virtualization as well, so a hypervisor can
debug the guest.

hmm, I read that as you as in favour of the patch?

I'd want some confirmation that it doesn't break anything, and that
there aren't any other places that need MSR_DE that this doesn't cover,
but in general yes.

Then you need to test drive the patch :)

I was thinking more along the lines of someone who's more familiar with
the relevant parts of the code confirming that it's really OK, not just
testing that it doesn't blow up in my face.

Still needs a test run, just throw it in :)