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

From: Joakim Tjernlund <hidden>
Date: 2012-07-20 08:37:21

Zang Roy-R61911 [off-list ref] wrote on 2012/07/20 10:27:52:

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-----Original Message-----
From: linuxppc-dev-bounces+tie-fei.zang=freescale.com@lists.ozlabs.org
[mailto:linuxppc-dev-bounces+tie-fei.zang=freescale.com@lists.ozlabs.org]
On Behalf Of Joakim Tjernlund
Sent: Friday, June 01, 2012 6:36 AM
To: Wood Scott-B07421
Cc: linuxppc-dev@ozlabs.org; Dan Malek; Bob Cochran; Support
Subject: Re: [RFC] [PATCH] powerpc: Add MSR_DE to MSR_KERNEL

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

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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,

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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

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is set, you will have problems with software debuggers that
utilize the the debugging registers in the chip itself.  You

only want

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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

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make use of the debug registers. Only if MSR_DE is set then

debug

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interrupts are generated. If a debug event leads to a debug

interrupt

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handled by a software debugger or if it leads to a debug halt

handled

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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

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hard data.
What I tried to mention is that handling the MSR_DE correct is not

only

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an emulator (JTAG debugger) requirement. Also a software debugger

may

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depend on a correct handled MSR_DE bit.

Yes, that made sense to me too. How would SW debuggers work if the

kernel keeps

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turning off MSR_DE first chance it gets?

The kernel selectively enables MSR_DE when it wants to debug.  I'm

not

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sure if anything will be bothered by leaving it on all the time.

This

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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.

It just occurred to me that you guys have this already in your Linux SDK so
it can't be that bad.

No. MSR_DE is ONLY added when using CW debug in SDK.
Roy

Yes, and I later found that user space debugging is busted if you turn on MSR_DE in
kernel.

 Jocke

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