Re: [PATCH 0/3] Add new ptrace request macros on PowerPC

[Michael Neuling <mikey@neuling.org>]

Anshuman Khandual <khandual@linux.vnet.ibm.com> wrote:

> On 04/29/2014 01:52 PM, Michael Neuling wrote:
> > That's not what that patch does. It shouldn't make any user visible changes
> > to DSCR or PPR.
> 
> It may not when it runs uninterrupted but after the tracee process has
> stopped, thread.dscr reflects the default DSCR value as mentioned
> before. This can be proved by changing the "dscr_default" value in
> arch/powerpc/sysfs.c file.

The intention with DSCR is that if the user changes the DSCR, the kernel
should always save/restore it.  If you are seeing something else, then
that is a bug.  Anton has a test case for this here:

  http://ozlabs.org/~anton/junkcode/dscr_explicit_test.c

If that is failing, then there is a bug that we need to fix.

The PPR is the same, except that the kernel can change it over a
syscall.

> > Over syscall PPR and DSCR may change.

Sorry, this should be only PPR.  DSCR shouldn't change over a syscall,
at least that's the intention.

> > Depending on your test case, that may
> > be your problem.
> 
> I would guess when the tracee process stops for ptrace analysis, tm_reclaim or
> tm_recheckpoint path might be crossed which is causing this dscr_default value
> to go into thread_struct.

That shouldn't happen.  If that's happening, it's a bug.

Mikey