Re: powerpc/process: Fix flush_all_to_thread for SPE

On Thu, 2018-10-25 at 12:07:54 UTC, "DATACOM - Felipe.Rechia" wrote:

From: "Felipe Rechia" <redacted>
Date: Wed, 24 Oct 2018 10:57:22 -0300
Subject: [PATCH] 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.

quoted

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 <redacted>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/e901378578c62202594cba0f6c076f

cheers