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

[Sagi Grimberg]

From: Shahaf Shuler <shahafs at 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 at gmail.com>
Signed-off-by: Shahaf Shuler <shahafs at mellanox.com>
Signed-off-by: Yongseok Koh <yskoh at mellanox.com>
Signed-off-by: Alexander Solganik <solganik at gmail.com>
Signed-off-by: Sagi Grimberg <sagi at 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);
 	ibv_fork_init();
 	rte_pci_register(&mlx5_driver);
 }
-- 
2.7.4