netdev - Re: [PATCH net] mlx5: avoid 64-bit division in dr_icm_pool_mr

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Date:   Wed, 2 Oct 2019 14:58:44 +0000
From:   Alex Vesker <valex@...lanox.com>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Leon Romanovsky <leon@...nel.org>
CC:     Borislav Petkov <bp@...en8.de>,
        Stephen Hemminger <stephen@...workplumber.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] mlx5: avoid 64-bit division in
 dr_icm_pool_mr_create()

On 10/2/2019 3:12 PM, Michal Kubecek wrote:
> Recently added code introduces 64-bit division in dr_icm_pool_mr_create()
> so that build on 32-bit architectures fails with
>
>    ERROR: "__umoddi3" [drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko] undefined!
>
> As the divisor is always a power of 2, we can use bitwise operation
> instead.
>
> Fixes: 29cf8febd185 ("net/mlx5: DR, ICM pool memory allocator")
> Reported-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/steering/dr_icm_pool.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/steering/dr_icm_pool.c b/drivers/net/ethernet/mellanox/mlx5/core/steering/dr_icm_pool.c
> index 913f1e5aaaf2..d7c7467e2d53 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/steering/dr_icm_pool.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/steering/dr_icm_pool.c
> @@ -137,7 +137,8 @@ dr_icm_pool_mr_create(struct mlx5dr_icm_pool *pool,
>   
>   	icm_mr->icm_start_addr = icm_mr->dm.addr;
>   
> -	align_diff = icm_mr->icm_start_addr % align_base;
> +	/* align_base is always a power of 2 */
> +	align_diff = icm_mr->icm_start_addr & (align_base - 1);
>   	if (align_diff)
>   		icm_mr->used_length = align_base - align_diff;
>   

Align diff is power of 2,  looks good to me.
Thanks for fixing it Michal.