Booting mpc85xx_defconfig kernel on QEMU leads to:

Bad trap at PC: fe9bab0, SR: 2d000, vector=800
awk[82]: unhandled trap (5) at 0 nip fe9bab0 lr fe9e01c code 5 in libc-2.27.so[fe5a000+17a000]
awk[82]: code: 3aa00000 3a800010 4bffe03c 9421fff0 7ca62b78 38a00000 93c10008 83c10008
awk[82]: code: 38210010 4bffdec8 9421ffc0 7c0802a6 <fc00048e> d8010008 4815190d 93810030
Trace/breakpoint trap
WARNING: no useful console

This is because allthough CONFIG_MATH_EMULATION is selected,
Exception 800 calls unknown_exception().

Call emulation_assist_interrupt() instead.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/kernel/head_85xx.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/head_85xx.S b/arch/powerpc/kernel/head_85xx.S
index 97e9ea0c7297..0f1641a31250 100644
--- a/arch/powerpc/kernel/head_85xx.S
+++ b/arch/powerpc/kernel/head_85xx.S
@@ -395,7 +395,7 @@ interrupt_base:
 #ifdef CONFIG_PPC_FPU
 	FP_UNAVAILABLE_EXCEPTION
 #else
-	EXCEPTION(0x0800, FP_UNAVAIL, FloatingPointUnavailable, unknown_exception)
+	EXCEPTION(0x0800, FP_UNAVAIL, FloatingPointUnavailable, emulation_assist_interrupt)
 #endif
 
 	/* System Call Interrupt */
-- 
2.41.0

On Mon, 25 Sep 2023 17:55:51 +0200, Christophe Leroy wrote:
> Booting mpc85xx_defconfig kernel on QEMU leads to:
> 
> Bad trap at PC: fe9bab0, SR: 2d000, vector=800
> awk[82]: unhandled trap (5) at 0 nip fe9bab0 lr fe9e01c code 5 in libc-2.27.so[fe5a000+17a000]
> awk[82]: code: 3aa00000 3a800010 4bffe03c 9421fff0 7ca62b78 38a00000 93c10008 83c10008
> awk[82]: code: 38210010 4bffdec8 9421ffc0 7c0802a6 <fc00048e> d8010008 4815190d 93810030
> Trace/breakpoint trap
> WARNING: no useful console
> 
> [...]

Applied to powerpc/fixes.

[1/1] powerpc/85xx: Fix math emulation exception
      https://git.kernel.org/powerpc/c/8e8a12ecbc86700b5e1a3596ce2b3c43dafad336

cheers

Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> Booting mpc85xx_defconfig kernel on QEMU leads to:

What machine/CPU model are you using for that?

I tried ppce500/e500 and that is working OK for me, ie. boots to
userspace and I see math instructions being emulated via the emulated
stats.

cheers

> Bad trap at PC: fe9bab0, SR: 2d000, vector=800
> awk[82]: unhandled trap (5) at 0 nip fe9bab0 lr fe9e01c code 5 in libc-2.27.so[fe5a000+17a000]
> awk[82]: code: 3aa00000 3a800010 4bffe03c 9421fff0 7ca62b78 38a00000 93c10008 83c10008
> awk[82]: code: 38210010 4bffdec8 9421ffc0 7c0802a6 <fc00048e> d8010008 4815190d 93810030
> Trace/breakpoint trap
> WARNING: no useful console
>
> This is because allthough CONFIG_MATH_EMULATION is selected,
> Exception 800 calls unknown_exception().
>
> Call emulation_assist_interrupt() instead.
>
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/kernel/head_85xx.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/head_85xx.S b/arch/powerpc/kernel/head_85xx.S
> index 97e9ea0c7297..0f1641a31250 100644
> --- a/arch/powerpc/kernel/head_85xx.S
> +++ b/arch/powerpc/kernel/head_85xx.S
> @@ -395,7 +395,7 @@ interrupt_base:
>  #ifdef CONFIG_PPC_FPU
>  	FP_UNAVAILABLE_EXCEPTION
>  #else
> -	EXCEPTION(0x0800, FP_UNAVAIL, FloatingPointUnavailable, unknown_exception)
> +	EXCEPTION(0x0800, FP_UNAVAIL, FloatingPointUnavailable, emulation_assist_interrupt)
>  #endif
>  
>  	/* System Call Interrupt */
> -- 
> 2.41.0