The tm-resched-dscr test has started failing sometimes, depending on
what compiler it's built with, eg:

  test: tm_resched_dscr
  Check DSCR TM context switch: tm-resched-dscr: tm-resched-dscr.c:76: test_body: Assertion `rv' failed.
  !! child died by signal 6

When it fails we see that the compiler doesn't initialise rv to 1 before
entering the inline asm block. Although that's counter intuitive, it
is allowed because we tell the compiler that the inline asm will write
to rv (using "=r"), meaning the original value is irrelevant.

Marking it as a read/write parameter would presumably work, but it seems
simpler to fix it by setting the initial value of rv in the inline asm.

Fixes: 96d01610 ("powerpc: Correct DSCR during TM context switch")
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Acked-by: NMichael Neuling <mikey@neuling.org>

Currently tbegin, tend etc are written as opcodes or asm instructions. So
standardise these to asm instructions.
Signed-off-by: NRashmica Gupta <rashmicy@gmail.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

This gives us standardised success/failure output and also handles
killing the test if it runs forever (2 minutes).
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

Correct the DSCR SPR becoming temporarily corrupted if a task is
context switched during a transaction.

The problem occurs while suspending the task and is caused by saving
the DSCR to thread.dscr after it has already been set to the CPU's
default value:

__switch_to() calls __switch_to_tm()
	which calls tm_reclaim_task()
	which calls tm_reclaim_thread()
	which calls tm_reclaim()
		where the DSCR is set to the CPU's default
__switch_to() calls _switch()
		where thread.dscr is set to the DSCR

When the task is resumed, it's transaction will be doomed (as usual)
and the DSCR SPR will be corrupted, although the checkpointed value
will be correct. Therefore the DSCR will be immediately corrected by
the transaction aborting, unless it has been suspended. In that case
the incorrect value can be seen by the task until it resumes the
transaction.

The fix is to treat the DSCR similarly to the TAR and save it early
in __switch_to().

A program exposing the problem is added to the kernel self tests as:
tools/testing/selftests/powerpc/tm/tm-resched-dscr.
Signed-off-by: NSam Bobroff <sam.bobroff@au1.ibm.com>
CC: <stable@vger.kernel.org> [v3.10+]
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>