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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
