Subject: Re: [PATCH 2/4] powerpc/64s: Add slb_full_bitmap rather than hard-coding U32_MAX

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 23 Jan 2019 14:32:38 +0530

Michael Ellerman <[email-masked]> writes:

> The recent rewrite of the SLB code into C included the assumption that
> all CPUs we run on have at least 32 SLB entries. This is currently
> true but a bit fragile as the SLB size is actually defined by the
> device tree and so could theoretically change at any time.
>
> The assumption is encoded in the fact that we use U32_MAX as the value
> for a full SLB bitmap. Instead, calculate what the full bitmap would
> be based on the SLB size we're given and store it. This still requires
> the SLB size to be a power of 2.

So if it is less than 32 we want to make sure we don't allocate an index
above 32. Is this the reason for that radom assert_slb_presence?

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

>
> Fixes: 126b11b294d1 ("powerpc/64s/hash: Add SLB allocation status bitmaps")
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  arch/powerpc/mm/slb.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
> index bc3914d54e26..61450a9cf30d 100644
> --- a/arch/powerpc/mm/slb.c
> +++ b/arch/powerpc/mm/slb.c
> @@ -506,9 +506,16 @@ void switch_slb(struct task_struct *tsk, struct mm_struct *mm)
>  	asm volatile("isync" : : : "memory");
>  }
>  
> +static u32 slb_full_bitmap;
> +
>  void slb_set_size(u16 size)
>  {
>  	mmu_slb_size = size;
> +
> +	if (size >= 32)
> +		slb_full_bitmap = U32_MAX;
> +	else
> +		slb_full_bitmap = (1ul << size) - 1;
>  }
>  
>  void slb_initialize(void)
> @@ -611,7 +618,7 @@ static enum slb_index alloc_slb_index(bool kernel)
>  	 * POWER7/8/9 have 32 SLB entries, this could be expanded if a
>  	 * future CPU has more.
>  	 */
> -	if (local_paca->slb_used_bitmap != U32_MAX) {
> +	if (local_paca->slb_used_bitmap != slb_full_bitmap) {
>  		index = ffz(local_paca->slb_used_bitmap);
>  		local_paca->slb_used_bitmap |= 1U << index;
>  		if (kernel)
> -- 
> 2.20.1