powerpc/process: Fix flush_all_to_thread for SPE

[Michael Ellerman]

On Thu, 2018-10-25 at 12:07:54 UTC, "DATACOM - Felipe.Rechia" wrote:
> From: "Felipe Rechia" <felipe.rechia at datacom.com.br>
> 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.
> 
> >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 at datacom.com.br>

Applied to powerpc next, thanks.

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

cheers