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

[Joakim Tjernlund]

Scott Wood <scottwood at freescale.com> wrote on 2012/05/31 19:47:53:
>
> On 05/31/2012 04:56 AM, Joakim Tjernlund wrote:
> > Abatron Support <support at abatron.ch> wrote on 2012/05/31 11:30:57:
> >>
> >>
> >>> Abatron Support <support at abatron.ch> wrote on 2012/05/30 14:08:26:
> >>>>
> >>>>>> 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.
> >>>>
> >>>>> 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.
> >>
> >>> 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?