Re: [PATCH 6/6] examples/vhost: add an option to enable Tx zero copy

Add an option, --tx-zero-copy, to enable Tx zero copy.

One thing worth noting while using Tx zero copy is the nb_tx_desc has
to be small enough so that the eth driver will hit the mbuf free
threshold easily and thus free mbuf more frequently.

The reason behind that is, when Tx zero copy is enabled, guest Tx used
vring will be updated only when corresponding mbuf is freed. If mbuf is
not freed frequently, the guest Tx vring could be starved.

Signed-off-by: Yuanhan Liu <redacted>
---
 examples/vhost/main.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/examples/vhost/main.c b/examples/vhost/main.c
index 9974f0b..e3437ad 100644
--- a/examples/vhost/main.c
+++ b/examples/vhost/main.c

@@ -130,6 +130,7 @@ static uint32_t enable_tx_csum;
 static uint32_t enable_tso;

 static int client_mode;
+static int tx_zero_copy;

 /* Specify timeout (in useconds) between retries on RX. */
 static uint32_t burst_rx_delay_time = BURST_RX_WAIT_US;

@@ -297,6 +298,17 @@ port_init(uint8_t port)

 	rx_ring_size = RTE_TEST_RX_DESC_DEFAULT;
 	tx_ring_size = RTE_TEST_TX_DESC_DEFAULT;
+
+	/*
+	 * When Tx zero copy is enabled, guest Tx used vring will be updated
+	 * only when corresponding mbuf is freed. Thus, the nb_tx_desc
+	 * (tx_ring_size here) must be small enough so that the driver will
+	 * hit the free threshold easily and free mbufs timely. Otherwise,
+	 * guest Tx vring would be starved.
+	 */
+	if (tx_zero_copy)
+		tx_ring_size = 64;