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

[Arseniy Krasnov <avkrasnov@salutedevices.com>]

On 30.11.2023 16:58, Michael S. Tsirkin wrote:
> 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 <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?
>>
>> >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.

I see, @Stefano, what do you think ?

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
>>>
>