Re: [PATCH v2 1/4] drm/panfrost: Simplify lock_region calculation

[Steven Price <steven.price@arm.com>]

On 24/08/2021 18:30, Alyssa Rosenzweig wrote:
> In lock_region, simplify the calculation of the region_width parameter.
> This field is the size, but encoded as ceil(log2(size)) - 1.
> ceil(log2(size)) may be computed directly as fls(size - 1). However, we
> want to use the 64-bit versions as the amount to lock can exceed
> 32-bits.
> 
> This avoids undefined (and completely wrong) behaviour when locking all
> memory (size ~0). In this case, the old code would "round up" ~0 to the
> nearest page, overflowing to 0. Since fls(0) == 0, this would calculate
> a region width of 10 + 0 = 10. But then the code would shift by
> (region_width - 11) = -1. As shifting by a negative number is undefined,
> UBSAN flags the bug. Of course, even if it were defined the behaviour is
> wrong, instead of locking all memory almost none would get locked.
> 
> The new form of the calculation corrects this special case and avoids
> the undefined behaviour.
> 
> Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
> Reported-and-tested-by: Chris Morgan <macromorgan@hotmail.com>
> Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
> Cc: <stable@vger.kernel.org>

Reviewed-by: Steven Price <steven.price@arm.com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +++++--------------
>  1 file changed, 5 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index 0da5b3100ab1..f6e02d0392f4 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -62,21 +62,12 @@ static void lock_region(struct panfrost_device *pfdev, u32 as_nr,
>  {
>  	u8 region_width;
>  	u64 region = iova & PAGE_MASK;
> -	/*
> -	 * fls returns:
> -	 * 1 .. 32
> -	 *
> -	 * 10 + fls(num_pages)
> -	 * results in the range (11 .. 42)
> -	 */
> -
> -	size = round_up(size, PAGE_SIZE);
>  
> -	region_width = 10 + fls(size >> PAGE_SHIFT);
> -	if ((size >> PAGE_SHIFT) != (1ul << (region_width - 11))) {
> -		/* not pow2, so must go up to the next pow2 */
> -		region_width += 1;
> -	}
> +	/* The size is encoded as ceil(log2) minus(1), which may be calculated
> +	 * with fls. The size must be clamped to hardware bounds.
> +	 */
> +	size = max_t(u64, size, PAGE_SIZE);
> +	region_width = fls64(size - 1) - 1;
>  	region |= region_width;
>  
>  	/* Lock the region that needs to be updated */
>