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.
Hmm.
Actually, let's think which packets require a response.
VIRTIO_VSOCK_OP_REQUEST
VIRTIO_VSOCK_OP_SHUTDOWN
VIRTIO_VSOCK_OP_CREDIT_REQUEST
the response to these always reports a state of an existing socket.
and, only one type of response is relevant for each socket.
So here's my suggestion:
stop queueing replies on the vsock device, instead,
simply store the response on the socket, and create a list of sockets
that have replies to be transmitted
WDYT?
> >
> > 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 <g...@amazon.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
> >
