Re: [Qemu-devel] [PATCH v2] vhost-vsock: report QMP event whensetrunning

[Stefan Hajnoczi]

On Fri, Dec 13, 2019 at 03:11:54PM +0800, address@hidden wrote:
> > This can be done efficiently as follows:
> > 1. kata-runtime listens on a vsock port
> > 2. kata-agent-port=PORT is added to the kernel command-line options
> > 3. kata-agent parses the port number and connects to the host
> > 
> > This eliminates the reconnection attempts.
> 
> There will be an additional problem if do this:
> Who decides which port the `runtime` should listen?

Let the host kernel automatically assign a port using VMADDR_PORT_ANY.
It works like this:

  struct sockaddr_vm svm = {
      .svm_family = AF_VSOCK,
      .svm_port = VMADDR_PORT_ANY,
      .svm_cid = VMADDR_CID_ANY,
  };

  int fd = socket(AF_VSOCK, SOCK_STREAM, 0);
  ...
  if (bind(fd, (const struct sockaddr *)&svm, sizeof(svm)) < 0) {
      ...
  }

  socklen_t socklen = sizeof(svm);
  if (getsockname(fd, (struct sockaddr *)&svm, &socklen) < 0) {
      ...
  }

  printf("cid %u port %u\n", svm.svm_cid, svm.svm_port);

> Consider the worst case: 
> The ports selected by two `runtime` running in parallel always conflict, 
> and this case is unavoidable, even if we can reduce the possibility of 
> conflicts through algorithms.
> Because we don't have a daemon that can allocate unique port to `runtime`.

The kernel assigns unique ports and only fails if the entire port
namespace is exhausted.  The port namespace is 32-bits so this is not a
real-world concern.

Does this information clarify how the runtime can connect to the guest
agent without loops or delays?

Stefan

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