11:34 a.m.
On Fri, Oct 18, 2019 at 10:58:43AM -0500, Eric Blake wrote:
On 10/18/19 10:39 AM, Richard W.M. Jones wrote:
>+Connect (synchronously) over the C<AF_VSOCK> protocol from a
>+virtual machine to an NBD server, usually running on the host. The
>+C<cid> and C<port> parameters specify the server address. Usually
>+C<cid> should be C<2> (to connect to the host), and C<port> might
be
>+C<10809> or another port number assigned to you by the host
>+administrator. This call returns when the connection has been made.";
You mentioned that right now, nbdkit has to be server on host, and
libnbd is client on guest. But if we can let nbdkit specify a cid,
doesn't this mean we can run nbdkit as server in guest, and then
connect libnbd as client on host? Then add 'nbdkit nbd vsock=...'
to let the nbdkit pass-through wrapper convert vsock from guest into
TCP or Unix socket on the host to other host clients that don't know
how to do vsock.
I'm afraid I don't understand this.
>@@ -2793,6 +2836,8 @@ let first_version = [
> "aio_connect_systemd_socket_activation", (1, 2);
> "connect_socket", (1, 2);
> "aio_connect_socket", (1, 2);
>+ "connect_vsock", (1, 2);
>+ "aio_connect_vsock", (1, 2);
As this is Linux-only (and for that matter, depends on kernel vsock
support), we probably need "supports_vsock" as an additional
function.
We could add it, but equally client code can just check AF_VSOCK the
same way that libnbd does, ie with a configure test for the header and
checking if AF_VSOCK is defined.
>+++ b/generator/states-connect.c
>@@ -37,6 +37,10 @@
> #include <sys/socket.h>
> #include <sys/un.h>
>+#ifdef HAVE_LINUX_VM_SOCKETS_H
>+#include <linux/vm_sockets.h>
>+#endif
>+
> /* Disable Nagle's algorithm on the socket, but don't fail. */
> static void
> disable_nagle (int sock)
>@@ -118,6 +122,24 @@ STATE_MACHINE {
> SET_NEXT_STATE (%^CONNECT.START);
> return 0;
>+ CONNECT_VSOCK.START:
>+#ifdef AF_VSOCK
>+ struct sockaddr_vm svm = {
>+ .svm_family = AF_VSOCK,
>+ .svm_cid = h->svm_cid,
>+ .svm_port = h->svm_port,
>+ };
Are there scenarios (mismatch in kernel vs. headers compiled
against, for instance) where compilation says AF_VSOCK exists but
where all attempts at vsock fail? If so, is there anything that we
should check dynamically, rather than just compile-time presence of
AF_VSOCK?
You can actually test this just by running the following command on
the host:
$ ./run strace -f -o /tmp/log nbdsh -c 'h.connect_vsock(2, 10809)'
The log shows how it fails:
3395572 socket(AF_VSOCK, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
...
3395572 connect(3, {sa_family=AF_VSOCK, sa_data="\0\09*\0\0\2\0\0\0\0\0\0\0"},
16) = -1 ENODEV (No such device)
That's unfortunate as it means we would have to connect to a
non-existent socket and determine if it returns ENODEV or ECONNREFUSED
(and if it returns something else - erm?!) Note that the socket(2)
syscall doesn't fail because creating a server socket is perfectly
valid whether in a host or guest context.
Rich.
But if nothing else, having:
int
libnbd_internal_supports_vsock(struct nbd_handle*h)
{
#ifdef AF_VSOCK
return 1;
#else
return 0;
}
is worth having.
Otherwise, the idea is pretty cool!
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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