Re: [RFC PATCH v4 2/2] MIPS: make FPU emulator optional

[Ralf Baechle <ralf@linux-mips.org>]

On Mon, Apr 07, 2014 at 02:53:15PM +0100, Paul Burton wrote:

> On Mon, Apr 07, 2014 at 12:57:04PM +0200, Manuel Lauss wrote:
> > This small patch makes the MIPS FPU emulator optional. The kernel
> > kills float-users on systems without a hardware FPU by sending a SIGILL.
> 
> One issue with this is that if someone runs a kernel with the FPU
> emulator disabled on hardware that has an FPU, they're likely to hit
> seemingly odd behaviour where FP works just fine until they hit a
> condition the hardware doesn't support. To make it clear that using FP
> without the emulator is a bad idea, perhaps it would be safer to disable
> FP entirely rather than only the emulator? Then userland can die the
> first time it uses FP instead of when it happens to operate on a
> denormal.
> 
> Unless there are FPUs which never generate an unimplemented operation
> exception, in which case perhaps more Kconfig is needed to identify such
> systems & allow the emulator to be disabled for those only.

The original reason for me to remove the FPU emulator option was that I
was getting flooded by bogus bug reports because users thought they could
remove the FPU emulator with a hardware FPU present.

Another pain point is that soft-FPU establishes another ABI variant so
I'd not mind to see soft-FPU go.

Some of the tradeoffs involved were a bit bogus at times.  Soft-fp
application code can be much bigger than hard-fp - to the point where the
50k or so for the kernel FP software are a good investment.

But I don't mind making the FPU emulator selectable again - but this
time with nasty kernel messages and killing of processes that happen to
dare to execute a FPU instruction.

  Ralf