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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 10 Aug 2023 04:51:59 -0400

On Thu, Aug 10, 2023 at 11:15:57AM +0800, Hawkins Jiawei wrote:
> 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 <[email-masked]>

Fixes: c7114b1249fa ("drivers/net/virtio_net: Added basic RSS support.")
Cc: Andrew Melnychenko <[email-masked]>
Acked-by: Michael S. Tsirkin <[email-masked]>

And this is stable material I believe.

> ---
> 
> 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