Configuring UAR as IO-mapped makes maximum throughput decline by noticeable
amount. If UAR is configured as write-combining register, a write memory
barrier is needed on ringing a doorbell. rte_wmb() is mostly effective when
the size of a burst is comparatively small. Revert the register back to
write-combining and enforce a write memory barrier instead, except for
vectorized Tx burst routines. Application can change it by setting
MLX5_SHUT_UP_BF under its own necessity.
Fixes: 9f9bebae5530 ("net/mlx5: don't map doorbell register to write combining")
Cc: sta...@dpdk.org
Cc: Sagi Grimberg <s...@grimberg.me>
Cc: Alexander Solganik <solga...@gmail.com>
Signed-off-by: Yongseok Koh <ys...@mellanox.com>
Acked-by: Shahaf Shuler <shah...@mellanox.com>
---
v2:
* Add documentation.
* Rename functions to minimize changes.
doc/guides/nics/mlx5.rst | 17 +++++++++++++++++
drivers/net/mlx5/mlx5.c | 2 --
drivers/net/mlx5/mlx5_rxtx.h | 23 +++++++++++++++++++++--
drivers/net/mlx5/mlx5_rxtx_vec_neon.h | 2 +-
drivers/net/mlx5/mlx5_rxtx_vec_sse.h | 2 +-
5 files changed, 40 insertions(+), 6 deletions(-)
diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index d24941a22..085d3940c 100644
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -171,6 +171,23 @@ Environment variables
This is disabled by default since this can also decrease performance for
unaligned packet sizes.
+- ``MLX5_SHUT_UP_BF``
+
+ Configures HW Tx doorbell register as IO-mapped.
+
+ By default, the HW Tx doorbell is configured as a write-combining register.
+ The register would be flushed to HW usually when the write-combining buffer
+ becomes full, but it depends on CPU design.
+
+ Except for vectorized Tx burst routines, a write memory barrier is enforced
+ after updating the register so that the update can be immediately visible to
+ HW.
+
+ When vectorized Tx burst is called, the barrier is set only if the burst size
+ is not aligned to MLX5_VPMD_TX_MAX_BURST. However, setting this environmental
+ variable will bring better latency even though the maximum throughput can
+ slightly decline.
+
Run-time configuration
~~~~~~~~~~~~~~~~~~~~~~
diff --git a/drivers/net/mlx5/mlx5.c b/drivers/net/mlx5/mlx5.c
index 89fdc134f..fcdcbc367 100644
--- a/drivers/net/mlx5/mlx5.c
+++ b/drivers/net/mlx5/mlx5.c
@@ -1037,8 +1037,6 @@ 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);
/* Match the size of Rx completion entry to the size of a cacheline. */
if (RTE_CACHE_LINE_SIZE == 128)
setenv("MLX5_CQE_SIZE", "128", 0);
diff --git a/drivers/net/mlx5/mlx5_rxtx.h b/drivers/net/mlx5/mlx5_rxtx.h
index ea037427b..d34f3cc04 100644
--- a/drivers/net/mlx5/mlx5_rxtx.h
+++ b/drivers/net/mlx5/mlx5_rxtx.h
@@ -578,15 +578,18 @@ mlx5_tx_mb2mr(struct mlx5_txq_data *txq, struct rte_mbuf
*mb)
}
/**
- * Ring TX queue doorbell.
+ * Ring TX queue doorbell and flush the update if requested.
*
* @param txq
* Pointer to TX queue structure.
* @param wqe
* Pointer to the last WQE posted in the NIC.
+ * @param cond
+ * Request for write memory barrier after BlueFlame update.
*/
static __rte_always_inline void
-mlx5_tx_dbrec(struct mlx5_txq_data *txq, volatile struct mlx5_wqe *wqe)
+mlx5_tx_dbrec_cond_wmb(struct mlx5_txq_data *txq, volatile struct mlx5_wqe
*wqe,
+ int cond)
{
uint64_t *dst = (uint64_t *)((uintptr_t)txq->bf_reg);
volatile uint64_t *src = ((volatile uint64_t *)wqe);
@@ -596,6 +599,22 @@ mlx5_tx_dbrec(struct mlx5_txq_data *txq, volatile struct
mlx5_wqe *wqe)
/* Ensure ordering between DB record and BF copy. */
rte_wmb();
*dst = *src;
+ if (cond)
+ rte_wmb();
+}
+
+/**
+ * Ring TX queue doorbell and flush the update by write memory barrier.
+ *
+ * @param txq
+ * Pointer to TX queue structure.
+ * @param wqe
+ * Pointer to the last WQE posted in the NIC.
+ */
+static __rte_always_inline void
+mlx5_tx_dbrec(struct mlx5_txq_data *txq, volatile struct mlx5_wqe *wqe)
+{
+ mlx5_tx_dbrec_cond_wmb(txq, wqe, 1);
}
#endif /* RTE_PMD_MLX5_RXTX_H_ */
diff --git a/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
b/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
index 4cb7f2889..61f5bc45b 100644
--- a/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
+++ b/drivers/net/mlx5/mlx5_rxtx_vec_neon.h
@@ -345,7 +345,7 @@ txq_burst_v(struct mlx5_txq_data *txq, struct rte_mbuf
**pkts, uint16_t pkts_n,
txq->wqe_ci += (nb_dword_in_hdr + pkts_n + (nb_dword_per_wqebb - 1)) /
nb_dword_per_wqebb;
/* Ring QP doorbell. */
- mlx5_tx_dbrec(txq, wqe);
+ mlx5_tx_dbrec_cond_wmb(txq, wqe, pkts_n < MLX5_VPMD_TX_MAX_BURST);
return pkts_n;
}
diff --git a/drivers/net/mlx5/mlx5_rxtx_vec_sse.h
b/drivers/net/mlx5/mlx5_rxtx_vec_sse.h
index e9819b762..a53027d84 100644
--- a/drivers/net/mlx5/mlx5_rxtx_vec_sse.h
+++ b/drivers/net/mlx5/mlx5_rxtx_vec_sse.h
@@ -344,7 +344,7 @@ txq_burst_v(struct mlx5_txq_data *txq, struct rte_mbuf
**pkts, uint16_t pkts_n,
txq->wqe_ci += (nb_dword_in_hdr + pkts_n + (nb_dword_per_wqebb - 1)) /
nb_dword_per_wqebb;
/* Ring QP doorbell. */
- mlx5_tx_dbrec(txq, wqe);
+ mlx5_tx_dbrec_cond_wmb(txq, wqe, pkts_n < MLX5_VPMD_TX_MAX_BURST);
return pkts_n;
}
--
2.11.0
