Re: [virtio-dev] [PATCH RESEND v2 2/2] virtio-net: use mtu size as buffe

Sorry for the delay. Didn't notice as this thread was not addressed to my work email. Please copy to my work email if it needs my immediate attention.

On 8/25/2022 5:38 AM, Gavin Li wrote:

Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for big
packets even when GUEST_* offloads are not present on the device.
However, if guest GSO is not supported, it would be sufficient to
allocate segments to cover just up the MTU size and no further.
Allocating the maximum amount of segments results in a large waste of
buffer space in the queue, which limits the number of packets that can
be buffered and can result in reduced performance.

Therefore, if guest GSO is not supported, use the MTU to calculate the
optimal amount of segments required.

When guest offload is enabled at runtime, RQ already has packets of bytes
less than 64K. So when packet of 64KB arrives, all the packets of such
size will be dropped. and RQ is now not usable.

So this means that during set_guest_offloads() phase, RQs have to be
destroyed and recreated, which requires almost driver reload.

Yes, this needs VIRTIO_F_RING_RESET and disable_vq_and_reset() to be done on RQ and refill it with appropriate size of buffer. Not for this patch anyway.

If VIRTIO_NET_F_CTRL_GUEST_OFFLOADS has been negotiated, then it should
always treat them as GSO enabled.

Below is the iperf TCP test results over a Mellanox NIC, using vDPA for
1 VQ, queue size 1024, before and after the change, with the iperf
server running over the virtio-net interface.

MTU(Bytes)/Bandwidth (Gbit/s)
             Before   After
  1500        22.5     22.4
  9000        12.8     25.9

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
---
changelog:
v1->v2
- Addressed comments from Jason, Michael, Si-Wei.
- Remove the flag of guest GSO support, set sg_num for big packets and
  use it directly
- Recalculate sg_num for big packets in virtnet_set_guest_offloads
- Replace the round up algorithm with DIV_ROUND_UP
---
 drivers/net/virtio_net.c | 41 +++++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e1904877d461..ec8c135a26d6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -225,6 +225,9 @@ struct virtnet_info {
 	/* I like... big packets and I cannot lie! */
 	bool big_packets;
 
+	/* number of sg entries allocated for big packets */
+	unsigned int big_packets_sg_num;
+
 	/* Host will merge rx buffers for big packets (shake it! shake it!) */
 	bool mergeable_rx_bufs;
 
@@ -1331,10 +1334,10 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
 	char *p;
 	int i, err, offset;
 
-	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
+	sg_init_table(rq->sg, vi->big_packets_sg_num + 2);
 
-	/* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */
-	for (i = MAX_SKB_FRAGS + 1; i > 1; --i) {
+	/* page in rq->sg[vi->big_packets_sg_num + 1] is list tail */
+	for (i = vi->big_packets_sg_num + 1; i > 1; --i) {
 		first = get_a_page(rq, gfp);
 		if (!first) {
 			if (list)
@@ -1365,7 +1368,7 @@ static int add_recvbuf_big(struct virtnet_info *vi, struct receive_queue *rq,
 
 	/* chain first in list head */
 	first->private = (unsigned long)list;
-	err = virtqueue_add_inbuf(rq->vq, rq->sg, MAX_SKB_FRAGS + 2,
+	err = virtqueue_add_inbuf(rq->vq, rq->sg, vi->big_packets_sg_num + 2,
 				  first, gfp);
 	if (err < 0)
 		give_pages(rq, first);
@@ -3690,13 +3693,31 @@ static bool virtnet_check_guest_gso(const struct virtnet_info *vi)
 		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO));
 }
 
+static void virtnet_set_big_packets_fields(struct virtnet_info *vi, const int mtu)
+{
+	bool guest_gso = virtnet_check_guest_gso(vi);
+
+	/* If we can receive ANY GSO packets, we must allocate large ones. */
+	if (mtu > ETH_DATA_LEN || guest_gso) {
+		vi->big_packets = true;
+		/* if the guest offload is offered by the device, user can modify
+		 * offload capability. In such posted buffers may short fall of size.
+		 * Hence allocate for max size.
+		 */
+		if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
+			vi->big_packets_sg_num = MAX_SKB_FRAGS;

MAX_SKB_FRAGS is needed when any of the guest_gso capability is offered. This is per spec regardless if VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is negotiated or not. Quoting spec:

If VIRTIO_NET_F_MRG_RXBUF is not negotiated:

If VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO are negotiated, the driver SHOULD populate the receive queue(s) with buffers of at least 65562 bytes.

I would just simply put:

vi->big_packets_sg_num = guest_gso ? MAX_SKB_FRAGS : DIV_ROUND_UP(mtu, PAGE_SIZE);

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