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
