Re: [PATCH 2/8] target/ppc: 405: Add missing exception handlers

[David Gibson]

On Fri, Jan 14, 2022 at 06:46:10PM -0300, Fabiano Rosas wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > On Mon, Jan 10, 2022 at 03:15:40PM -0300, Fabiano Rosas wrote:
> >> Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
> >> ---
> >>  target/ppc/cpu_init.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >> 
> >> diff --git a/target/ppc/cpu_init.c b/target/ppc/cpu_init.c
> >> index a50ddaeaae..9097948e67 100644
> >> --- a/target/ppc/cpu_init.c
> >> +++ b/target/ppc/cpu_init.c
> >> @@ -1951,7 +1951,9 @@ static void init_excp_4xx_softmmu(CPUPPCState *env)
> >>      env->excp_vectors[POWERPC_EXCP_EXTERNAL] = 0x00000500;
> >>      env->excp_vectors[POWERPC_EXCP_ALIGN]    = 0x00000600;
> >>      env->excp_vectors[POWERPC_EXCP_PROGRAM]  = 0x00000700;
> >> +    env->excp_vectors[POWERPC_EXCP_FPU]      = 0x00000800;
> >
> > I have a vague recollection from my days of working on 405 that there
> > may have been something funky with FP emulation on there: e.g. FP
> > instructions causing 0x700 program interrupts instead of FP unavailble
> > interrupts or something.
> 
> Maybe this (from the manual):
> 
>   Program - causing conditions:
>   
>   Attempted execution of illegal instructions, TRAP instruction,
>   privileged instruction in problem state, or auxiliary processor (APU)
>   instruction, or unimplemented FPU instruction, or unimplemented APU
>   instruction, or APU interrupt, or FPU interrupt
>   
>   FPU Unavailable - causing conditions:
>   
>   Attempted execution of an FPU instruction when MSR[FP]=0.
> 
> There's also this bit:
> 
>   Attempted execution of an APU instruction while the APUc405exception
>   signal is asserted) results in a program interrupt. Similarly, attempted
>   execution of an FPU instruction whilethe FPUc405exception signal is
>   asserted) also results in a program interrupt. The following also result
>   in program interrupts: attempted execution of an APU instruction while
>   APUc405DcdAPUOp is asserted but APUC405DcdValidOp is deasserted; and
>   attempted execution of an FPU instruction while APUc405DcdFpuOp but
>   APUC405DcdValidOp is deasserted.

Right... those do seem to suggest that FP comes in as a 0x700 rather
than 0x800.  Really hard to be sure without checking an actual chip.

> > I might be remembering incorrectly - the manual does seem to imply
> > that 0x800 FP unavailable is there as normal, but it might be worth
> > double checking this (against real hardware, if possible).
> 
> The Linux kernel has the vectors that I'm adding disabled:
> 
>   EXCEPTION(0x0800, Trap_08, unknown_exception) <-- FPU
>   EXCEPTION(0x0900, Trap_09, unknown_exception)
>   EXCEPTION(0x0A00, Trap_0A, unknown_exception) 
>   EXCEPTION(0x0B00, Trap_0B, unknown_exception)
>   ...
>   EXCEPTION(0x0F00, Trap_0F, unknown_exception) <-- APU
> 
> (0xf20 would probably cause a crash as we'd jump to the middle of the
> exception prologue)

Right.. that's fairly strong evidence that those vectors don't operate
in practice.

> Maybe I should drop this patch then? That way future developers won't
> feel tempted to raise one of these.

Maybe.  Better yet would be to verify on a chip then drop a comment in
there explicitly describing the situation

> It seems mostly inconsequential either way, what do you think?

Well, yes.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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