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

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:
> 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.

I thought we'd convinced ourselves it didn't matter in practice due to
the proximity of the entry/exit.

>> 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.

That will be the common case (no update), so yes I agree.

cheers