Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT

Michael S. Tsirkin
2023-11-30T13:42:58Z

On Thu, Nov 30, 2023 at 04:08:39PM +0300, Arseniy Krasnov wrote:
> Send credit update message when SO_RCVLOWAT is updated and it is bigger
> than number of bytes in rx queue. It is needed, because 'poll()' will
> wait until number of bytes in rx queue will be not smaller than
> SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup
> for tx/rx is possible: sender waits for free space and receiver is
> waiting data in 'poll()'.
>
> Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
> ---
> Changelog:
> v1 -> v2:
> * Update commit message by removing 'This patch adds XXX' manner.
> * Do not initialize 'send_update' variable - set it directly during
> first usage.
> v3 -> v4:
> * Fit comment in 'virtio_transport_notify_set_rcvlowat()' to 80 chars.
> v4 -> v5:
> * Do not change callbacks order in transport structures.
>
> drivers/vhost/vsock.c | 1 +
> include/linux/virtio_vsock.h | 1 +
> net/vmw_vsock/virtio_transport.c | 1 +
> net/vmw_vsock/virtio_transport_common.c | 27 +++++++++++++++++++++++++
> net/vmw_vsock/vsock_loopback.c | 1 +
> 5 files changed, 31 insertions(+)
>
> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> index f75731396b7e..4146f80db8ac 100644
> --- a/drivers/vhost/vsock.c
> +++ b/drivers/vhost/vsock.c
> @@ -451,6 +451,7 @@ static struct virtio_transport vhost_transport = {
> .notify_buffer_size = virtio_transport_notify_buffer_size,
>
> .read_skb = virtio_transport_read_skb,
> + .notify_set_rcvlowat = virtio_transport_notify_set_rcvlowat
> },
>
> .send_pkt = vhost_transport_send_pkt,
> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> index ebb3ce63d64d..c82089dee0c8 100644
> --- a/include/linux/virtio_vsock.h
> +++ b/include/linux/virtio_vsock.h
> @@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit);
> void virtio_transport_deliver_tap_pkt(struct sk_buff *skb);
> int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list);
> int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor);
> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val);
> #endif /* _LINUX_VIRTIO_VSOCK_H */
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index af5bab1acee1..8007593a3a93 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -539,6 +539,7 @@ static struct virtio_transport virtio_transport = {
> .notify_buffer_size = virtio_transport_notify_buffer_size,
>
> .read_skb = virtio_transport_read_skb,
> + .notify_set_rcvlowat = virtio_transport_notify_set_rcvlowat
> },
>
> .send_pkt = virtio_transport_send_pkt,
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index f6dc896bf44c..1cb556ad4597 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -1684,6 +1684,33 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
> }
> EXPORT_SYMBOL_GPL(virtio_transport_read_skb);
>
> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val)
> +{
> + struct virtio_vsock_sock *vvs = vsk->trans;
> + bool send_update;
> +
> + spin_lock_bh(&vvs->rx_lock);
> +
> + /* If number of available bytes is less than new SO_RCVLOWAT value,
> + * kick sender to send more data, because sender may sleep in its
> + * 'send()' syscall waiting for enough space at our side.
> + */
> + send_update = vvs->rx_bytes < val;
> +
> + spin_unlock_bh(&vvs->rx_lock);
> +
> + if (send_update) {
> + int err;
> +
> + err = virtio_transport_send_credit_update(vsk);
> + if (err < 0)
> + return err;
> + }
> +
> + return 0;
> +}

I find it strange that this will send a credit update
even if nothing changed since this was called previously.
I'm not sure whether this is a problem protocol-wise,
but it certainly was not envisioned when the protocol was
built. WDYT?

> +EXPORT_SYMBOL_GPL(virtio_transport_notify_set_rcvlowat);
> +
> MODULE_LICENSE("GPL v2");
> MODULE_AUTHOR("Asias He");
> MODULE_DESCRIPTION("common code for virtio vsock");
> diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
> index 048640167411..9f4b814fbbc7 100644
> --- a/net/vmw_vsock/vsock_loopback.c
> +++ b/net/vmw_vsock/vsock_loopback.c
> @@ -98,6 +98,7 @@ static struct virtio_transport loopback_transport = {
> .notify_buffer_size = virtio_transport_notify_buffer_size,
>
> .read_skb = virtio_transport_read_skb,
> + .notify_set_rcvlowat = virtio_transport_notify_set_rcvlowat
> },
>
> .send_pkt = vsock_loopback_send_pkt,
> --
> 2.25.1