[PATCH] powerpc/book3s64/kuap: SPRN_AMR modification need CSI instructions before and after

[Nicholas Piggin]

Excerpts from Nicholas Piggin's message of April 20, 2020 10:17 am:
> Excerpts from Aneesh Kumar K.V's message of April 19, 2020 11:53 pm:
>> As per the ISA, context synchronizing instructions is needed before and after
>> SPRN_AMR update. Use isync before and the CSI after is implied by the rfid
>> that we will use to switch to a new context.
> 
> Not entirely sure if we need this. This will restore AMR to more 
> permissive, so if it executes ahead of a stray load from this
> context, it won't make it fault.
> 
> That said, leaving this end open makes it harder to reason about
> user access protection I guess, so let's add it.

We probably should test whether it needs updating, like the entry 
code does.

Thanks,
Nick