Re: [Qemu-devel] [PATCH 01/13] PPC: Allow Rc bit to be set on mtspr

[Thomas Huth <thuth@redhat.com>]

On 23/10/15 15:56, Mark Cave-Ayland wrote:
> From: Alexander Graf <agraf@suse.de>
> 
> According to the ISA setting the Rc bit on mtspr is undefined behavior.
> Real 750 hardware simply ignores the bit and doesn't touch cr0 though.

According to PowerISA 2.07, chapter 1.1.3:

"Reserved fields in instructions are ignored by the pro-
cessor."

And the lowest bit of the mtspr opcode is marked as reserved. So I think
this is not just a hack, but even a proper fix.

> Unfortunately, Mac OS 9 relies on this fact and executes a few mtspr
> instructions (to set XER for example) with Rc set.
> 
> So let's handle the bit the same way hardware does and ignore it.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>  target-ppc/translate.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> index c2bc1a7..d1f0f13 100644
> --- a/target-ppc/translate.c
> +++ b/target-ppc/translate.c
> @@ -9884,7 +9884,7 @@ GEN_HANDLER(mtcrf, 0x1F, 0x10, 0x04, 0x00000801, PPC_MISC),
>  GEN_HANDLER(mtmsrd, 0x1F, 0x12, 0x05, 0x001EF801, PPC_64B),
>  #endif
>  GEN_HANDLER(mtmsr, 0x1F, 0x12, 0x04, 0x001FF801, PPC_MISC),
> -GEN_HANDLER(mtspr, 0x1F, 0x13, 0x0E, 0x00000001, PPC_MISC),
> +GEN_HANDLER(mtspr, 0x1F, 0x13, 0x0E, 0x00000000, PPC_MISC),
>  GEN_HANDLER(dcbf, 0x1F, 0x16, 0x02, 0x03C00001, PPC_CACHE),
>  GEN_HANDLER(dcbi, 0x1F, 0x16, 0x0E, 0x03E00001, PPC_CACHE),
>  GEN_HANDLER(dcbst, 0x1F, 0x16, 0x01, 0x03E00001, PPC_CACHE),

Reviewed-by: Thomas Huth <thuth@redhat.com>