Re: [PATCH v3] target/ppc: Fix fallback to MFSS for MFFS* instructions on pre 3.0 ISAs

[Richard Henderson <richard.henderson@linaro.org>]

On 5/10/23 12:19, Richard Purdie wrote:
> The following commits changed the code such that the fallback to MFSS for MFFSCRN,
> MFFSCRNI, MFFSCE and MFFSL on pre 3.0 ISAs was removed and became an illegal instruction:
> 
>    bf8adfd88b547680aa857c46098f3a1e94373160 - target/ppc: Move mffscrn[i] to decodetree
>    394c2e2fda70da722f20fb60412d6c0ca4bfaa03 - target/ppc: Move mffsce to decodetree
>    3e5bce70efe6bd1f684efbb21fd2a316cbf0657e - target/ppc: Move mffsl to decodetree
> 
> The hardware will handle them as a MFFS instruction as the code did previously.
> This means applications that were segfaulting under qemu when encountering these
> instructions which is used in glibc libm functions for example.
> 
> The fallback for MFFSCDRN and MFFSCDRNI added in a later patch was also missing.
> 
> This patch restores the fallback to MFSS for these instructions on pre 3.0s ISAs
> as the hardware decoder would, fixing the segfaulting libm code. It doesn't have
> the fallback for 3.0 onwards to match hardware behaviour.
> 
> Signed-off-by: Richard Purdie<richard.purdie@linuxfoundation.org>
> ---
>   target/ppc/insn32.decode           | 20 +++++++++++++-------
>   target/ppc/translate/fp-impl.c.inc | 22 ++++++++++++++++------
>   2 files changed, 29 insertions(+), 13 deletions(-)
> 
> v3 - drop fallback to MFFS for 3.0 ISA to match hardware
> v2 - switch to use decodetree pattern groups per feedback

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~