Re: Hyper-V vsock streams do not fill the supplied buffer in full

[Stefano Garzarella]

Hi Gary,

On Wed, Jul 5, 2023 at 12:45 AM Gary Guo <gary@xxxxxxxxxxx> wrote:
>
> When a vsock stream is called with recvmsg with a buffer, it only fills
> the buffer with data from the first single VM packet. Even if there are
> more VM packets at the time and the buffer is still not completely
> filled, it will just leave the buffer partially filled.
>
> This causes some issues when in WSLD which uses the vsock in
> non-blocking mode and uses epoll.
>
> For stream-oriented sockets, the epoll man page [1] says that
>
> > For stream-oriented files (e.g., pipe, FIFO, stream socket),
> > the condition that the read/write I/O space is exhausted can
> > also be detected by checking the amount of data read from /
> > written to the target file descriptor.  For example, if you
> > call read(2) by asking to read a certain amount of data and
> > read(2) returns a lower number of bytes, you can be sure of
> > having exhausted the read I/O space for the file descriptor.
>
> This has been used as an optimisation in the wild for reducing number
> of syscalls required for stream sockets (by asserting that the socket
> will not have to polled to EAGAIN in edge-trigger mode, if the buffer
> given to recvmsg is not filled completely). An example is Tokio, which
> starting in v1.21.0 [2].
>
> When this optimisation combines with the behaviour of Hyper-V vsock, it
> causes issue in this scenario:
> * the VM host send data to the guest, and it's splitted into multiple
>   VM packets
> * sk_data_ready is called and epoll returns, notifying the userspace
>   that the socket is ready
> * userspace call recvmsg with a buffer, and it's partially filled
> * userspace assumes that the stream socket is depleted, and if new data
>   arrives epoll will notify it again.
> * kernel always considers the socket to be ready, and since it's in
>   edge-trigger mode, the epoll instance will never be notified again.
>
> This different realisation of the readiness causes the userspace to
> block forever.

Thanks for the detailed description of the problem.

I think we should fix the hvs_stream_dequeue() in
net/vmw_vsock/hyperv_transport.c.
We can do something similar to what we do in
virtio_transport_stream_do_dequeue() in
net/vmw_vsock/virtio_transport_common.c

@Dexuan WDYT?

Thanks,
Stefano