powerpc/process: Fix flush_all_to_thread for SPE
authorFelipe Rechia <felipe.rechia@datacom.com.br>
Wed, 24 Oct 2018 13:57:22 +0000 (10:57 -0300)
committerMichael Ellerman <mpe@ellerman.id.au>
Fri, 26 Oct 2018 10:58:58 +0000 (21:58 +1100)
commite901378578c62202594cba0f6c076f3df365ec91
tree1b16ca7e9f1075a5cc6104c3650e93e87d844c83
parent8dce6b2215eaa91dbf04463e11098a48748da5ab
powerpc/process: Fix flush_all_to_thread for SPE

Fix a bug introduced by the creation of flush_all_to_thread() for
processors that have SPE (Signal Processing Engine) and use it to
compute floating-point operations.

>From userspace perspective, the problem was seen in attempts of
computing floating-point operations which should generate exceptions.
For example:

  fork();
  float x = 0.0 / 0.0;
  isnan(x);           // forked process returns False (should be True)

The operation above also should always cause the SPEFSCR FINV bit to
be set. However, the SPE floating-point exceptions were turned off
after a fork().

Kernel versions prior to the bug used flush_spe_to_thread(), which
first saves SPEFSCR register values in tsk->thread and then calls
giveup_spe(tsk).

After commit 579e633e764e, the save_all() function was called first
to giveup_spe(), and then the SPEFSCR register values were saved in
tsk->thread. This would save the SPEFSCR register values after
disabling SPE for that thread, causing the bug described above.

Fixes 579e633e764e ("powerpc: create flush_all_to_thread()")
Signed-off-by: Felipe Rechia <felipe.rechia@datacom.com.br>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
arch/powerpc/kernel/process.c