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

[Arseniy Krasnov]

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 <avkrasnov@xxxxxxxxxxxxxxxxx>
>> ---
>>  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

> 
> 
>> +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
>