Re: CVS commit: src/sys/arch/m68k/m68k

[Tetsuya Isaki <isaki%pastel-flower.jp@localhost>]

At Sun, 30 Mar 2025 04:44:26 +0000,
Nathanial Sloss wrote:
> Module Name:	src
> Committed By:	nat
> Date:		Sun Mar 30 04:44:26 UTC 2025
> 
> Modified Files:
> 	src/sys/arch/m68k/m68k: switch_subr.s
> 
> Log Message:
> Sprinkle nop.
> 
> This is the kernel part of addressing the issue with fpu emulation on lc040
> cpus.

The originator of this PR port-mac68k/13078 said (in 2001):
| Workaround: place a NOP before the F-line instruction.

And you responded (in Nov 2024):
| The submitter is indeed right adding a nop before an fpu
| instruction does fix the fpu emulation on the lc040...

But you add a NOP before RET.  Why?

Furthermore,

> --- switch_subr.s   31 Oct 2024 07:30:28 -0000  1.38
> +++ switch_subr.s   30 Mar 2025 04:44:26 -0000  1.39
> @@ -80,6 +80,7 @@
>  ASENTRY_NOPROFILE(cpu_idle)
>         stop    #PSL_LOWIPL
>  GLOBAL(_Idle)                          /* For sun2/sun3's clock.c ... */
> +       nop
>         rts

68LC040 runs this part but there are no FPU instructions.
Is your 68LC040 affected if you remove this NOP?

> @@ -207,6 +208,7 @@
>  .Lcpu_switch_nofprest:
>         movl    %d1,%d0                 | return outgoing lwp
>         movl    %d0,%a0                 | (in a0, too)
> +       nop
>         rts

This is in cpu_switchto.
68LC040 runs this part but never encounters FPU instructions.
Is your 68LC040 affected if you remove this NOP?

> @@ -255,6 +257,7 @@
>  #endif /* FPCOPROC */
>  #endif /* !_M68K_CUSTOM_FPU_CTX */
>         moveq   #0,%d0                  | return 0
> +       nop
>         rts

This is in savectx.
68LC040 runs this part but never encounters FPU instructions.
Is your 68LC040 affected if you remove this NOP?

> @@ -286,6 +289,7 @@
>         frestore (%sp)
>         fnop
>         addql   #4,%sp
> +       nop
>         rts
>  #endif

This is in m68k_make_fpu_idle_frame().
68LC040 doesn't seem to call it.
Is your 68LC040 affected if you remove this NOP?

> @@ -307,6 +311,7 @@
>         fmovem  %fp0-%fp7,FPF_REGS(%a0) | save FP general registers
>         fmovem  %fpcr/%fpsr/%fpi,FPF_FPCR(%a0) | save FP control registers
>  .Lm68881sdone:
> +       nop
>         rts
>  #endif
>  #if defined(M68060)
> @@ -318,6 +323,7 @@
>         fmovem  %fpsr,FPF_FPSR(%a0)
>         fmovem  %fpi,FPF_FPI(%a0)
>  .Lm68060sdone:
> +       nop
>          rts
>  #endif

These are in m68881_save().
68LC040 doesn't seem to call it.  In addition, the second chunk
is not compiled because M68060 is not defined on mac68k.
Is your 68LC040 affected if you remove these NOPs?

> @@ -335,6 +341,7 @@
>         fmovem  FPF_REGS(%a0),%fp0-%fp7 | restore FP general registers
>  .Lm68881rdone:
>         frestore (%a0)                  | restore state
> +       nop
>         rts
>  #endif
>  #if defined(M68060)
> @@ -347,6 +354,7 @@
>         fmovem  FPF_REGS(%a0),%fp0-%fp7 | restore FP general registers
>  .Lm68060fprdone:
>         frestore (%a0)                  | restore state
> +       nop
>         rts
>  #endif
>  #endif

These are in m68881_restore().  Same as above.

You may be observing something different...
---
Tetsuya Isaki <isaki%pastel-flower.jp@localhost / isaki%NetBSD.org@localhost>