Re: [dpdk-dev] [PATCH 2/2] net/mlx5: don't map doorbell register to write combining

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/21/2017 8:47 AM, Sagi Grimberg wrote:
> From: Shahaf Shuler <shahafs@mellanox.com>
> 
> By default, Verbs maps the doorbell register to write combining.
> Working with write combining is useful for drivers which use blue flame
> for the doorbell write.
> 
> Since mlx5 PMD uses only doorbells and write combining mapping requires
> an extra memory barrier to flush the doorbell after its write, setting
> the mapping to un-cached by default.
> 
> Such change is reduces the max and average round trip
> latency significantly.
> 
> Reported-by: Alexander Solganik <solganik@gmail.com>
> Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> Signed-off-by: Alexander Solganik <solganik@gmail.com>
> Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
> ---
>  drivers/net/mlx5/mlx5.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/mlx5/mlx5.c b/drivers/net/mlx5/mlx5.c
> index b7e5046325c0..4c2a0b9652e3 100644
> --- a/drivers/net/mlx5/mlx5.c
> +++ b/drivers/net/mlx5/mlx5.c
> @@ -920,6 +920,8 @@ rte_mlx5_pmd_init(void)
>  	 * using this PMD, which is not supported in forked processes.
>  	 */
>  	setenv("RDMAV_HUGEPAGES_SAFE", "1", 1);
> +	/* Don't map UAR to WC if BlueFlame is not used.*/
> +	setenv("MLX5_SHUT_UP_BF", "1", 1);

Although technically this is possible, I wonder how good idea it is a
driver communicating with other processes via system environment variable?

>  	ibv_fork_init();
>  	rte_pci_register(&mlx5_driver);
>  }
>