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

[Nicholas Piggin]

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.

Thanks,
Nick

> 
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
> ---
>  arch/powerpc/include/asm/book3s/64/kup-radix.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/kup-radix.h b/arch/powerpc/include/asm/book3s/64/kup-radix.h
> index 3bcef989a35d..224658efe2fd 100644
> --- a/arch/powerpc/include/asm/book3s/64/kup-radix.h
> +++ b/arch/powerpc/include/asm/book3s/64/kup-radix.h
> @@ -16,6 +16,7 @@
>  #ifdef CONFIG_PPC_KUAP
>  	BEGIN_MMU_FTR_SECTION_NESTED(67)
>  	ld	\gpr, STACK_REGS_KUAP(r1)
> +	isync
>  	mtspr	SPRN_AMR, \gpr
>  	END_MMU_FTR_SECTION_NESTED_IFSET(MMU_FTR_RADIX_KUAP, 67)
>  #endif
> @@ -62,8 +63,14 @@
>  
>  static inline void kuap_restore_amr(struct pt_regs *regs)
>  {
> -	if (mmu_has_feature(MMU_FTR_RADIX_KUAP))
> +	if (mmu_has_feature(MMU_FTR_RADIX_KUAP)) {
> +		isync();
>  		mtspr(SPRN_AMR, regs->kuap);
> +		/*
> +		 * No following isync/CSI required because we will be
> +		 * returning to a different context using rfid
> +		 */
> +	}
>  }
>  
>  static inline void kuap_check_amr(void)
> -- 
> 2.25.2
> 
>