Message-ID: <5l2xvaamvvoayrz3vlof4rarp5hkeig5sljhsqzypo6xx4fcip@3bhjnwe5g7ha>
Date: Thu, 3 Apr 2025 10:24:30 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Stefan Hajnoczi <stefanha@...hat.com>, 
	Alexander Graf <graf@...zon.com>, "Michael S . Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	virtualization@...ts.linux.dev, kvm@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>, 
	Jakub Kicinski <kuba@...nel.org>, Eric Dumazet <edumazet@...gle.com>, 
	"David S . Miller" <davem@...emloft.net>, nh-open-source@...zon.com
Subject: Re: [PATCH v2] vsock/virtio: Remove queued_replies pushback logic

On Wed, Apr 02, 2025 at 12:14:24PM -0400, Stefan Hajnoczi wrote:
>On Tue, Apr 01, 2025 at 08:13:49PM +0000, Alexander Graf wrote:
>> Ever since the introduction of the virtio vsock driver, it included
>> pushback logic that blocks it from taking any new RX packets until the
>> TX queue backlog becomes shallower than the virtqueue size.
>>
>> This logic works fine when you connect a user space application on the
>> hypervisor with a virtio-vsock target, because the guest will stop
>> receiving data until the host pulled all outstanding data from the VM.
>>
>> With Nitro Enclaves however, we connect 2 VMs directly via vsock:
>>
>>   Parent      Enclave
>>
>>     RX -------- TX
>>     TX -------- RX
>>
>> This means we now have 2 virtio-vsock backends that both have the pushback
>> logic. If the parent's TX queue runs full at the same time as the
>> Enclave's, both virtio-vsock drivers fall into the pushback path and
>> no longer accept RX traffic. However, that RX traffic is TX traffic on
>> the other side which blocks that driver from making any forward
>> progress. We're now in a deadlock.
>>
>> To resolve this, let's remove that pushback logic altogether and rely on
>> higher levels (like credits) to ensure we do not consume unbounded
>> memory.
>
>The reason for queued_replies is that rx packet processing may emit tx
>packets. Therefore tx virtqueue space is required in order to process
>the rx virtqueue.
>
>queued_replies puts a bound on the amount of tx packets that can be
>queued in memory so the other side cannot consume unlimited memory. Once
>that bound has been reached, rx processing stops until the other side
>frees up tx virtqueue space.
>
>It's been a while since I looked at this problem, so I don't have a
>solution ready. In fact, last time I thought about it I wondered if the
>design of virtio-vsock fundamentally suffers from deadlocks.
>
>I don't think removing queued_replies is possible without a replacement
>for the bounded memory and virtqueue exhaustion issue though. Credits
>are not a solution - they are about socket buffer space, not about
>virtqueue space, which includes control packets that are not accounted
>by socket buffer space.

This is a very good point that I missed, I need to add a comment in the 
code to explain it, because it wasn't clear to me! Thank you very much 
Stefan!

So, IIUC, with this patch, a host or a sibling VM (e.g.  enclave, 
parent), can flood the VM with requests like VIRTIO_VSOCK_OP_REQUEST 
(even for example with a random port that is not open) that require a 
response. If the peer that is sending the requests, using the RX 
virtqueue, does not consume the TX virtqueue, it easily causes a 
consumption of all the memory on the other peer, which initially starts 
filling up the TX virtqueue, but when it becomes full starts using the 
internal queue indiscriminately.

I agree, if we want to get rid of queued_replies, we should find some 
other way to avoid this. So far I can't think of anything other than to 
stop the consumption of the virtqueue and wait for the other peer to 
consume the other one.

Any other ideas?

Thanks,
Stefano


>
>>
>> RX and TX queues share the same work queue. To prevent starvation of TX
>> by an RX flood and vice versa now that the pushback logic is gone, let's
>> deliberately reschedule RX and TX work after a fixed threshold (256) of
>> packets to process.
>>
>> Fixes: 0ea9e1d3a9e3 ("VSOCK: Introduce virtio_transport.ko")
>> Signed-off-by: Alexander Graf <graf@...zon.com>
>> ---
>>  net/vmw_vsock/virtio_transport.c | 70 +++++++++-----------------------
>>  1 file changed, 19 insertions(+), 51 deletions(-)
>>
>> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
>> index f0e48e6911fc..54030c729767 100644
>> --- a/net/vmw_vsock/virtio_transport.c
>> +++ b/net/vmw_vsock/virtio_transport.c
>> @@ -26,6 +26,12 @@ static struct virtio_vsock __rcu *the_virtio_vsock;
>>  static DEFINE_MUTEX(the_virtio_vsock_mutex); /* protects the_virtio_vsock */
>>  static struct virtio_transport virtio_transport; /* forward declaration */
>>
>> +/*
>> + * Max number of RX packets transferred before requeueing so we do
>> + * not starve TX traffic because they share the same work queue.
>> + */
>> +#define VSOCK_MAX_PKTS_PER_WORK 256
>> +
>>  struct virtio_vsock {
>>  	struct virtio_device *vdev;
>>  	struct virtqueue *vqs[VSOCK_VQ_MAX];
>> @@ -44,8 +50,6 @@ struct virtio_vsock {
>>  	struct work_struct send_pkt_work;
>>  	struct sk_buff_head send_pkt_queue;
>>
>> -	atomic_t queued_replies;
>> -
>>  	/* The following fields are protected by rx_lock.  vqs[VSOCK_VQ_RX]
>>  	 * must be accessed with rx_lock held.
>>  	 */
>> @@ -158,7 +162,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>  		container_of(work, struct virtio_vsock, send_pkt_work);
>>  	struct virtqueue *vq;
>>  	bool added = false;
>> -	bool restart_rx = false;
>> +	int pkts = 0;
>>
>>  	mutex_lock(&vsock->tx_lock);
>>
>> @@ -172,6 +176,12 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>  		bool reply;
>>  		int ret;
>>
>> +		if (++pkts > VSOCK_MAX_PKTS_PER_WORK) {
>> +			/* Allow other works on the same queue to run */
>> +			queue_work(virtio_vsock_workqueue, work);
>> +			break;
>> +		}
>> +
>>  		skb = virtio_vsock_skb_dequeue(&vsock->send_pkt_queue);
>>  		if (!skb)
>>  			break;
>> @@ -184,17 +194,6 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>  			break;
>>  		}
>>
>> -		if (reply) {
>> -			struct virtqueue *rx_vq = vsock->vqs[VSOCK_VQ_RX];
>> -			int val;
>> -
>> -			val = atomic_dec_return(&vsock->queued_replies);
>> -
>> -			/* Do we now have resources to resume rx processing? */
>> -			if (val + 1 == virtqueue_get_vring_size(rx_vq))
>> -				restart_rx = true;
>> -		}
>> -
>>  		added = true;
>>  	}
>>
>> @@ -203,9 +202,6 @@ virtio_transport_send_pkt_work(struct work_struct *work)
>>
>>  out:
>>  	mutex_unlock(&vsock->tx_lock);
>> -
>> -	if (restart_rx)
>> -		queue_work(virtio_vsock_workqueue, &vsock->rx_work);
>>  }
>>
>>  /* Caller need to hold RCU for vsock.
>> @@ -261,9 +257,6 @@ virtio_transport_send_pkt(struct sk_buff *skb)
>>  	 */
>>  	if (!skb_queue_empty_lockless(&vsock->send_pkt_queue) ||
>>  	    virtio_transport_send_skb_fast_path(vsock, skb)) {
>> -		if (virtio_vsock_skb_reply(skb))
>> -			atomic_inc(&vsock->queued_replies);
>> -
>>  		virtio_vsock_skb_queue_tail(&vsock->send_pkt_queue, skb);
>>  		queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
>>  	}
>> @@ -277,7 +270,7 @@ static int
>>  virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>>  {
>>  	struct virtio_vsock *vsock;
>> -	int cnt = 0, ret;
>> +	int ret;
>>
>>  	rcu_read_lock();
>>  	vsock = rcu_dereference(the_virtio_vsock);
>> @@ -286,17 +279,7 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>>  		goto out_rcu;
>>  	}
>>
>> -	cnt = virtio_transport_purge_skbs(vsk, &vsock->send_pkt_queue);
>> -
>> -	if (cnt) {
>> -		struct virtqueue *rx_vq = vsock->vqs[VSOCK_VQ_RX];
>> -		int new_cnt;
>> -
>> -		new_cnt = atomic_sub_return(cnt, &vsock->queued_replies);
>> -		if (new_cnt + cnt >= virtqueue_get_vring_size(rx_vq) &&
>> -		    new_cnt < virtqueue_get_vring_size(rx_vq))
>> -			queue_work(virtio_vsock_workqueue, &vsock->rx_work);
>> -	}
>> +	virtio_transport_purge_skbs(vsk, &vsock->send_pkt_queue);
>>
>>  	ret = 0;
>>
>> @@ -367,18 +350,6 @@ static void virtio_transport_tx_work(struct work_struct *work)
>>  		queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
>>  }
>>
>> -/* Is there space left for replies to rx packets? */
>> -static bool virtio_transport_more_replies(struct virtio_vsock *vsock)
>> -{
>> -	struct virtqueue *vq = vsock->vqs[VSOCK_VQ_RX];
>> -	int val;
>> -
>> -	smp_rmb(); /* paired with atomic_inc() and atomic_dec_return() */
>> -	val = atomic_read(&vsock->queued_replies);
>> -
>> -	return val < virtqueue_get_vring_size(vq);
>> -}
>> -
>>  /* event_lock must be held */
>>  static int virtio_vsock_event_fill_one(struct virtio_vsock *vsock,
>>  				       struct virtio_vsock_event *event)
>> @@ -613,6 +584,7 @@ static void virtio_transport_rx_work(struct work_struct *work)
>>  	struct virtio_vsock *vsock =
>>  		container_of(work, struct virtio_vsock, rx_work);
>>  	struct virtqueue *vq;
>> +	int pkts = 0;
>>
>>  	vq = vsock->vqs[VSOCK_VQ_RX];
>>
>> @@ -627,11 +599,9 @@ static void virtio_transport_rx_work(struct work_struct *work)
>>  			struct sk_buff *skb;
>>  			unsigned int len;
>>
>> -			if (!virtio_transport_more_replies(vsock)) {
>> -				/* Stop rx until the device processes already
>> -				 * pending replies.  Leave rx virtqueue
>> -				 * callbacks disabled.
>> -				 */
>> +			if (++pkts > VSOCK_MAX_PKTS_PER_WORK) {
>> +				/* Allow other works on the same queue to run */
>> +				queue_work(virtio_vsock_workqueue, work);
>>  				goto out;
>>  			}
>>
>> @@ -675,8 +645,6 @@ static int virtio_vsock_vqs_init(struct virtio_vsock *vsock)
>>  	vsock->rx_buf_max_nr = 0;
>>  	mutex_unlock(&vsock->rx_lock);
>>
>> -	atomic_set(&vsock->queued_replies, 0);
>> -
>>  	ret = virtio_find_vqs(vdev, VSOCK_VQ_MAX, vsock->vqs, vqs_info, NULL);
>>  	if (ret < 0)
>>  		return ret;
>> --
>> 2.47.1
>>

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