[PATCH] virtio-net: Zero max_tx_vq field for VIRTIO_NET_CTRL_MQ_HASH_CONFIG case

[Hawkins Jiawei <yin31149@gmail.com>]

Kernel uses `struct virtio_net_ctrl_rss` to save command-specific-data
for both the VIRTIO_NET_CTRL_MQ_HASH_CONFIG and
VIRTIO_NET_CTRL_MQ_RSS_CONFIG commands.

According to the VirtIO standard, "Field reserved MUST contain zeroes.
It is defined to make the structure to match the layout of
virtio_net_rss_config structure, defined in 5.1.6.5.7.".

Yet for the VIRTIO_NET_CTRL_MQ_HASH_CONFIG command case, the `max_tx_vq`
field in struct virtio_net_ctrl_rss, which corresponds to the
`reserved` field in struct virtio_net_hash_config, is not zeroed,
thereby violating the VirtIO standard.

This patch solves this problem by zeroing this field in
virtnet_init_default_rss().

Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
---

TestStep
========
1. Boot QEMU with one virtio-net-pci net device with `mq` and `hash`
feature on, command line like:
      -netdev tap,vhost=off,...
      -device virtio-net-pci,mq=on,hash=on,...

2. Trigger VIRTIO_NET_CTRL_MQ_HASH_CONFIG command in guest, command
line like:
	ethtool -K eth0 rxhash on

Without this patch, in virtnet_commit_rss_command(), we can see the
`max_tx_vq` field is 1 in gdb like below:

	pwndbg> p vi->ctrl->rss
	$1 = {
	  hash_types = 63,
	  indirection_table_mask = 0,
	  unclassified_queue = 0,
	  indirection_table = {0 <repeats 128 times>},
	  max_tx_vq = 1,
	  hash_key_length = 40 '(',
	  ...
	}

With this patch, in virtnet_commit_rss_command(), we can see the
`max_tx_vq` field is 0 in gdb like below:

	pwndbg> p vi->ctrl->rss
	$1 = {
	  hash_types = 63,
	  indirection_table_mask = 0,
	  unclassified_queue = 0,
	  indirection_table = {0 <repeats 128 times>},
	  max_tx_vq = 0,
	  hash_key_length = 40 '(',
	  ...
	}

 drivers/net/virtio_net.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 1270c8d23463..8db38634ae82 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2761,7 +2761,7 @@ static void virtnet_init_default_rss(struct virtnet_info *vi)
 		vi->ctrl->rss.indirection_table[i] = indir_val;
 	}
 
-	vi->ctrl->rss.max_tx_vq = vi->curr_queue_pairs;
+	vi->ctrl->rss.max_tx_vq = vi->has_rss ? vi->curr_queue_pairs : 0;
 	vi->ctrl->rss.hash_key_length = vi->rss_key_size;
 
 	netdev_rss_key_fill(vi->ctrl->rss.key, vi->rss_key_size);
-- 
2.34.1