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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Nov 2023 08:58:58 -0500

On Thu, Nov 30, 2023 at 04:43:34PM +0300, Arseniy Krasnov wrote:
> 
> 
> On 30.11.2023 16:42, Michael S. Tsirkin wrote:
> > 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 <[email-masked]>
> >> ---
> >>  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?
> 
> >From virtio spec I found:
> 
> It is also valid to send a VIRTIO_VSOCK_OP_CREDIT_UPDATE packet without previously receiving a
> VIRTIO_VSOCK_OP_CREDIT_REQUEST packet. This allows communicating updates any time a change
> in buffer space occurs.
> So I guess there is no limitations to send such type of packet, e.g. it is not
> required to be a reply for some another packet. Please, correct me if im wrong.
> 
> Thanks, Arseniy

Absolutely. My point was different - with this patch it is possible
that you are not adding any credits at all since the previous
VIRTIO_VSOCK_OP_CREDIT_UPDATE.

> 
> > 
> > 
> >> +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
> >