[PATCH 1/3] powerpc: Complete FSCR context switch

[Jack Miller]

> I'm not sure that works on processes before power8.
>
> There DSCR SPR number 0x11 will always trap and emulate from userspace
> (see arch/powerpc/kernel/traps.c:emulate_instruction()).  That is not
> controlled by FSCR and should work on POWER7 where FSCR is not
> present.  We need to set the inherit bit there too.
>
> DSCR SPR number 0x3 is controlled by fscr, but it's only avaliable on
> POWER8.
>
>> Right now the FSCR switch is conditional on FTR_ARCH_207S which is
>> more exclusive than FTR_DSCR, but I guess the actual FSCR register is
>> universal to PPC64 like the fscr field in the thread struct? If so, I
>> can just move the FSCR save/restore out of the 207 conditional.
>
> FSCR was only introduced in power8, so it needs to be 207 conditional
>

So on P6/P7 (which have FTR_DSCR) set we're potentially mtspr'ing to a
non-existent register? Yuck. Can at least strip that logic out thanks
to the full context switch, I think, even if dscr_inherit actually has
a use.

- Jack