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Date:   Mon, 28 Dec 2020 16:36:35 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        jasowang@...hat.com, Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH rfc 1/3] virtio-net: support transmit hash report

On Mon, Dec 28, 2020 at 11:22:31AM -0500, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> Virtio-net supports sharing the flow hash from host to guest on rx.
> Do the same on transmit, to allow the host to infer connection state
> for more robust routing and telemetry.
> 
> Linux derives ipv6 flowlabel and ECMP multipath from sk->sk_txhash,
> and updates these fields on error with sk_rethink_txhash. This feature
> allows the host to make similar decisions.
> 
> Besides the raw hash, optionally also convey connection state for
> this hash. Specifically, the hash rotates on transmit timeout. To
> avoid having to keep a stateful table in the host to detect flow
> changes, explicitly notify when a hash changed due to timeout.
> 
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  drivers/net/virtio_net.c        | 24 +++++++++++++++++++++---
>  include/uapi/linux/virtio_net.h | 10 +++++++++-
>  2 files changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 21b71148c532..b917b7333928 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -201,6 +201,9 @@ struct virtnet_info {
>  	/* Host will merge rx buffers for big packets (shake it! shake it!) */
>  	bool mergeable_rx_bufs;
>  
> +	/* Guest will pass tx path info to the host */
> +	bool has_tx_hash;
> +
>  	/* Has control virtqueue */
>  	bool has_cvq;
>  
> @@ -394,9 +397,9 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  
>  	hdr_len = vi->hdr_len;
>  	if (vi->mergeable_rx_bufs)
> -		hdr_padded_len = sizeof(*hdr);
> +		hdr_padded_len = max_t(unsigned int, hdr_len, sizeof(*hdr));
>  	else
> -		hdr_padded_len = sizeof(struct padded_vnet_hdr);
> +		hdr_padded_len = ALIGN(hdr_len, 16);
>  
>  	/* hdr_valid means no XDP, so we can copy the vnet header */
>  	if (hdr_valid)
> @@ -1534,6 +1537,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  	struct virtio_net_hdr_mrg_rxbuf *hdr;
>  	const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
>  	struct virtnet_info *vi = sq->vq->vdev->priv;
> +	struct virtio_net_hdr_v1_hash *ht;
>  	int num_sg;
>  	unsigned hdr_len = vi->hdr_len;
>  	bool can_push;
> @@ -1558,6 +1562,14 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  	if (vi->mergeable_rx_bufs)
>  		hdr->num_buffers = 0;
>  
> +	ht = (void *)hdr;
> +	if (vi->has_tx_hash) {
> +		ht->hash_value = cpu_to_virtio32(vi->vdev, skb->hash);
> +		ht->hash_report = skb->l4_hash ? VIRTIO_NET_HASH_REPORT_L4 :
> +						 VIRTIO_NET_HASH_REPORT_OTHER;
> +		ht->hash_state = VIRTIO_NET_HASH_STATE_DEFAULT;
> +	}
> +
>  	sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
>  	if (can_push) {
>  		__skb_push(skb, hdr_len);
> @@ -3054,6 +3066,11 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	else
>  		vi->hdr_len = sizeof(struct virtio_net_hdr);
>  
> +	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_TX_HASH)) {
> +		vi->has_tx_hash = true;
> +		vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash);
> +	}
> +
>  	if (virtio_has_feature(vdev, VIRTIO_F_ANY_LAYOUT) ||
>  	    virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
>  		vi->any_header_sg = true;
> @@ -3243,7 +3260,8 @@ static struct virtio_device_id id_table[] = {
>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
>  	VIRTIO_NET_F_MTU, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS, \
> -	VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY
> +	VIRTIO_NET_F_SPEED_DUPLEX, VIRTIO_NET_F_STANDBY, \
> +	VIRTIO_NET_F_TX_HASH
>  
>  static unsigned int features[] = {
>  	VIRTNET_FEATURES,
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index 3f55a4215f11..f6881b5b77ee 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -57,6 +57,7 @@
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
>  
> +#define VIRTIO_NET_F_TX_HASH	  56	/* Guest sends hash report */
>  #define VIRTIO_NET_F_HASH_REPORT  57	/* Supports hash report */
>  #define VIRTIO_NET_F_RSS	  60	/* Supports RSS RX steering */
>  #define VIRTIO_NET_F_RSC_EXT	  61	/* extended coalescing info */
> @@ -170,8 +171,15 @@ struct virtio_net_hdr_v1_hash {
>  #define VIRTIO_NET_HASH_REPORT_IPv6_EX         7
>  #define VIRTIO_NET_HASH_REPORT_TCPv6_EX        8
>  #define VIRTIO_NET_HASH_REPORT_UDPv6_EX        9
> +#define VIRTIO_NET_HASH_REPORT_L4              10
> +#define VIRTIO_NET_HASH_REPORT_OTHER           11

Need to specify these I guess ...
Can't there be any consistency with RX hash?
Handy for VM2VM ...


>  	__le16 hash_report;
> -	__le16 padding;
> +	union {
> +		__le16 padding;
> +#define VIRTIO_NET_HASH_STATE_DEFAULT          0
> +#define VIRTIO_NET_HASH_STATE_TIMEOUT_BIT      0x1
> +		__le16 hash_state;
> +	};
>  };
>  
>  #ifndef VIRTIO_NET_NO_LEGACY
> -- 
> 2.29.2.729.g45daf8777d-goog

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