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

[Joakim Tjernlund]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote on 2012/06/01 11:12:51:
>
> On Thu, 2012-05-31 at 11:05 +0200, Joakim Tjernlund wrote:
> > 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.
>
> The kernel normally controls when to set or not set MSR:DE, at least
> when using SW breakpoints. Setting it globally should remain some kind
> of specific debug option.
>
> In fact on some CPUs, we even leave user set dbcr settings and rely on
> DE being off in kernel space to avoid user->kernel attacks via the debug
> registers (I think we still do that on 64-bit BookE though it should
> eventually change).

hmm, would it not be better to always clear out/control dbcr settings and always have MSR:DE
on? It would be much easier to control dbcr, even dynamically, than MSR:DE

 Jocke