Subject: Re: [PATCH] powerpc/64s/hash: Fix assert_slb_presence() use of the slbfee. instruction

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Fri, 15 Feb 2019 15:57:34 +0530

Nicholas Piggin <[email-masked]> writes:

> The slbfee. instruction must have bit 24 of RB clear, failure to do
> so can result in false negatives that result in incorrect assertions.
>
> This is not obvious from the ISA v3.0B document, which only says:
>
>     The hardware ignores the contents of RB 36:38 40:63 -- p.1032
>
> This patch fixes the bug and also clears all other bits from PPC bit
> 36-63, which is good practice when dealing with reserved or ignored
> bits.
>

Reviewed-by: Aneesh Kumar K.V <[email-masked]>

> Fixes: e15a4fea4d ("powerpc/64s/hash: Add some SLB debugging tests")
> Reported-by: Aneesh Kumar K.V <[email-masked]>
> Tested-by: Aneesh Kumar K.V <[email-masked]>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>  arch/powerpc/mm/slb.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
> index bc3914d54e26..5986df48359b 100644
> --- a/arch/powerpc/mm/slb.c
> +++ b/arch/powerpc/mm/slb.c
> @@ -69,6 +69,11 @@ static void assert_slb_presence(bool present, unsigned long ea)
>  	if (!cpu_has_feature(CPU_FTR_ARCH_206))
>  		return;
>  
> +	/*
> +	 * slbfee. requires bit 24 (PPC bit 39) be clear in RB. Hardware
> +	 * ignores all other bits from 0-27, so just clear them all.
> +	 */
> +	ea &= ~((1UL << 28) - 1);

I guess these numbers '28' are derived from the size of the smallest
segment we support. If co can we use ESID_MASK?

>  	asm volatile(__PPC_SLBFEE_DOT(%0, %1) : "=r"(tmp) : "r"(ea) : "cr0");
>  
>  	WARN_ON(present == (tmp == 0));
> -- 
> 2.18.0