Re: [PATCH v2] vsock/virtio: Remove queued_replies pushback logic

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Apr 04, 2025 at 10:04:38AM +0200, Alexander Graf wrote:
> 
> On 03.04.25 14:21, Michael S. Tsirkin wrote:
> > 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?
> 
> 
> Wouldn't that create the same problem again? The socket will eventually push
> back any new data that it can take because its FIFO is full. At that point,
> the "other side" could still have a queue full of requests on exactly that
> socket that need to get processed. We can now not pull those packets off the
> virtio queue, because we can not enqueue responses.

Either I don't understand what you wrote or I did not explain myself
clearly. 

In this idea there needs to be a single response enqueued
like this in the socket, because, no more than one ever needs to
be outstanding per socket.

For example, until VIRTIO_VSOCK_OP_REQUEST
is responded to, the socket is not active and does not need to
send anything.


> 
> But that means now the one queue is blocked from making forward progress,
> because we are applying back pressure. And that means everything can grind
> to a halt and we have the same deadlock this patch is trying to fix.
> 
> I don't see how we can possibly guarantee a lossless data channel over a
> tiny wire (single, fixed size, in order virtio ring) while also guaranteeing
> bounded memory usage. One of the constraints need to go: Either we are no
> longer lossless or we effectively allow unbounded memory usage.
> 
> 
> Alex