Re: freezes when not emulating CPU (was: MIDI input patch)

[Bart Oldeman <bartoldeman@users.sourceforge.net>]

Hello Stas,

> Which means that the comment in
> do_vm86.c about fnsave is wrong, and the
> code that restores the dosemu FPU state
> should likely to be returned. You can,
> for example, memset the vm86_fpu_state
> to 0 in fpu_reset() and see the SIGFPE
> coming from all around the sound code
> (no other code use FPU in dosemu).

I've been looking at this. The SIGFPEs don't happen for i386 (default
FP), only for x86-64 or when explicitly compiling with SSE floating
point (using gcc -msse2 -mfpmath=sse). The "fnsave" instruction really
re-initializes the FPU (it's documented), but with SSE one needs to
use "fxsave" and "fninit", and that still doesn't reinitialize the
SIMD part. Adding an "ldmxcsr" instruction solved that problem.

I still think that a simple FPU environment reset is sufficient, no
need to restore all the registers, because the DOSEMU FPU code is not
interrupted by DOS code.

The fpu save/restore operations aren't cheap, so perhaps one could
only use them around the sound code instead of for every vm86 call.
Though that could be messy.

> Also, I found no docs about this FPU
> init/reset stuff, so everything in this
> patch is just a wild guesses based on
> a look into a bochs code.
> Also, there is a need to add the handling
> for exception 0x13 (SIMD FPE), which is
> what the SIGFPE is about today. I mean,
> at least print_exception_info() should
> write something meaningfull about SIGFPE
> instead of "Unknown exception".

I've done that now.

Bart