[dpdk-stable] patch 'net/virtio: fix performance regression due to TSO' has been queued to stable release 16.11.1

[Yuanhan Liu]

Hi,

FYI, your patch has been queued to stable release 16.11.1

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable
yet. It will be pushed if I get no objections before 02/18/17.
So please shout if anyone has objections.

Thanks.

	--yliu

---
>From 43ce94d1a20a60e7a0406bc6586379b23bf5bd18 Mon Sep 17 00:00:00 2001
From: Yuanhan Liu <yuanhan.liu at linux.intel.com>
Date: Wed, 11 Jan 2017 12:27:11 +0800
Subject: [PATCH] net/virtio: fix performance regression due to TSO

[ upstream commit c9ea670c1dc7e3f111d8139f915082b60c9c1ffe ]

TSO is now enabled, but it's not actually being used by default in a
simple L2 forward mode. In such case, we have to zero the virtio net
headers, to inform the vhost backend that no offload is being used:

    hdr->csum_start = 0;
    hdr->csum_offset = 0;
    hdr->flags = 0;

    hdr->gso_type = 0;
    hdr->gso_size = 0;
    hdr->hdr_len = 0;

Such writes could be very costly; it introduces severe cache issues:
The above operations introduce cache write for each packet, which
stalls the read operation from the vhost backend.

The fact that virtio net header is initiated to zero in PMD driver
init stage means that these costly writes are unnecessary and could
be avoided:

    if (hdr->csum_start != 0)
        hdr->csum_start = 0;

And that's what the macro ASSIGN_UNLESS_EQUAL does. With this, the
performance drop introduced by TSO enabling is recovered: it could
be up to 20% in micro benchmarking.

Fixes: 58169a9c8153 ("net/virtio: support Tx checksum offload")
Fixes: 696573046e9e ("net/virtio: support TSO")

Cc: Olivier Matz <olivier.matz at 6wind.com>
Cc: Maxime Coquelin <maxime.coquelin at redhat.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>
Reviewed-by: Olivier Matz <olivier.matz at 6wind.com>
---
 drivers/net/virtio/virtio_rxtx.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
index 22d97a4..edbd3cd 100644
--- a/drivers/net/virtio/virtio_rxtx.c
+++ b/drivers/net/virtio/virtio_rxtx.c
@@ -258,6 +258,12 @@ tx_offload_enabled(struct virtio_hw *hw)
 		vtpci_with_feature(hw, VIRTIO_NET_F_HOST_TSO6);
 }
 
+/* avoid write operation when necessary, to lessen cache issues */
+#define ASSIGN_UNLESS_EQUAL(var, val) do {	\
+	if ((var) != (val))			\
+		(var) = (val);			\
+} while (0)
+
 static inline void
 virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
 		       uint16_t needed, int use_indirect, int can_push)
@@ -337,9 +343,9 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
 			break;
 
 		default:
-			hdr->csum_start = 0;
-			hdr->csum_offset = 0;
-			hdr->flags = 0;
+			ASSIGN_UNLESS_EQUAL(hdr->csum_start, 0);
+			ASSIGN_UNLESS_EQUAL(hdr->csum_offset, 0);
+			ASSIGN_UNLESS_EQUAL(hdr->flags, 0);
 			break;
 		}
 
@@ -355,9 +361,9 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie,
 				cookie->l3_len +
 				cookie->l4_len;
 		} else {
-			hdr->gso_type = 0;
-			hdr->gso_size = 0;
-			hdr->hdr_len = 0;
+			ASSIGN_UNLESS_EQUAL(hdr->gso_type, 0);
+			ASSIGN_UNLESS_EQUAL(hdr->gso_size, 0);
+			ASSIGN_UNLESS_EQUAL(hdr->hdr_len, 0);
 		}
 	}
 
-- 
1.9.0