Re: [PATCH RFC net-next v5 03/14] af_vsock: support multi-transport datagrams

From: Stefano Garzarella
Date: Thu Aug 03 2023 - 08:43:24 EST

On Thu, Aug 03, 2023 at 12:53:22AM +0000, Bobby Eshleman wrote:
On Wed, Aug 02, 2023 at 10:24:44PM +0000, Bobby Eshleman wrote:
On Sun, Jul 23, 2023 at 12:53:15AM +0300, Arseniy Krasnov wrote:
>
>
> On 19.07.2023 03:50, Bobby Eshleman wrote:
> > This patch adds support for multi-transport datagrams.
> >
> > This includes:
> > - Per-packet lookup of transports when using sendto(sockaddr_vm)
> > - Selecting H2G or G2H transport using VMADDR_FLAG_TO_HOST and CID in
> > sockaddr_vm
> > - rename VSOCK_TRANSPORT_F_DGRAM to VSOCK_TRANSPORT_F_DGRAM_FALLBACK
> > - connect() now assigns the transport for (similar to connectible
> > sockets)
> >
> > To preserve backwards compatibility with VMCI, some important changes
> > are made. The "transport_dgram" / VSOCK_TRANSPORT_F_DGRAM is changed to
> > be used for dgrams only if there is not yet a g2h or h2g transport that
> > has been registered that can transmit the packet. If there is a g2h/h2g
> > transport for that remote address, then that transport will be used and
> > not "transport_dgram". This essentially makes "transport_dgram" a
> > fallback transport for when h2g/g2h has not yet gone online, and so it
> > is renamed "transport_dgram_fallback". VMCI implements this transport.
> >
> > The logic around "transport_dgram" needs to be retained to prevent
> > breaking VMCI:
> >
> > 1) VMCI datagrams existed prior to h2g/g2h and so operate under a
> > different paradigm. When the vmci transport comes online, it registers
> > itself with the DGRAM feature, but not H2G/G2H. Only later when the
> > transport has more information about its environment does it register
> > H2G or G2H. In the case that a datagram socket is created after
> > VSOCK_TRANSPORT_F_DGRAM registration but before G2H/H2G registration,
> > the "transport_dgram" transport is the only registered transport and so
> > needs to be used.
> >
> > 2) VMCI seems to require a special message be sent by the transport when a
> > datagram socket calls bind(). Under the h2g/g2h model, the transport
> > is selected using the remote_addr which is set by connect(). At
> > bind time there is no remote_addr because often no connect() has been
> > called yet: the transport is null. Therefore, with a null transport
> > there doesn't seem to be any good way for a datagram socket to tell the
> > VMCI transport that it has just had bind() called upon it.
> >
> > With the new fallback logic, after H2G/G2H comes online the socket layer
> > will access the VMCI transport via transport_{h2g,g2h}. Prior to H2G/G2H
> > coming online, the socket layer will access the VMCI transport via
> > "transport_dgram_fallback".
> >
> > Only transports with a special datagram fallback use-case such as VMCI
> > need to register VSOCK_TRANSPORT_F_DGRAM_FALLBACK.
> >
> > Signed-off-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>
> > ---
> > drivers/vhost/vsock.c | 1 -
> > include/linux/virtio_vsock.h | 2 --
> > include/net/af_vsock.h | 10 +++---
> > net/vmw_vsock/af_vsock.c | 64 ++++++++++++++++++++++++++-------
> > net/vmw_vsock/hyperv_transport.c | 6 ----
> > net/vmw_vsock/virtio_transport.c | 1 -
> > net/vmw_vsock/virtio_transport_common.c | 7 ----
> > net/vmw_vsock/vmci_transport.c | 2 +-
> > net/vmw_vsock/vsock_loopback.c | 1 -
> > 9 files changed, 58 insertions(+), 36 deletions(-)
> >
> > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > index ae8891598a48..d5d6a3c3f273 100644
> > --- a/drivers/vhost/vsock.c
> > +++ b/drivers/vhost/vsock.c
> > @@ -410,7 +410,6 @@ static struct virtio_transport vhost_transport = {
> > .cancel_pkt = vhost_transport_cancel_pkt,
> >
> > .dgram_enqueue = virtio_transport_dgram_enqueue,
> > - .dgram_bind = virtio_transport_dgram_bind,
> > .dgram_allow = virtio_transport_dgram_allow,
> >
> > .stream_enqueue = virtio_transport_stream_enqueue,
> > diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > index 18cbe8d37fca..7632552bee58 100644
> > --- a/include/linux/virtio_vsock.h
> > +++ b/include/linux/virtio_vsock.h
> > @@ -211,8 +211,6 @@ void virtio_transport_notify_buffer_size(struct vsock_sock *vsk, u64 *val);
> > u64 virtio_transport_stream_rcvhiwat(struct vsock_sock *vsk);
> > bool virtio_transport_stream_is_active(struct vsock_sock *vsk);
> > bool virtio_transport_stream_allow(u32 cid, u32 port);
> > -int virtio_transport_dgram_bind(struct vsock_sock *vsk,
> > - struct sockaddr_vm *addr);
> > bool virtio_transport_dgram_allow(u32 cid, u32 port);
> >
> > int virtio_transport_connect(struct vsock_sock *vsk);
> > diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> > index 305d57502e89..f6a0ca9d7c3e 100644
> > --- a/include/net/af_vsock.h
> > +++ b/include/net/af_vsock.h
> > @@ -96,13 +96,13 @@ struct vsock_transport_send_notify_data {
> >
> > /* Transport features flags */
> > /* Transport provides host->guest communication */
> > -#define VSOCK_TRANSPORT_F_H2G 0x00000001
> > +#define VSOCK_TRANSPORT_F_H2G 0x00000001
> > /* Transport provides guest->host communication */
> > -#define VSOCK_TRANSPORT_F_G2H 0x00000002
> > -/* Transport provides DGRAM communication */
> > -#define VSOCK_TRANSPORT_F_DGRAM 0x00000004
> > +#define VSOCK_TRANSPORT_F_G2H 0x00000002
> > +/* Transport provides fallback for DGRAM communication */
> > +#define VSOCK_TRANSPORT_F_DGRAM_FALLBACK 0x00000004
> > /* Transport provides local (loopback) communication */
> > -#define VSOCK_TRANSPORT_F_LOCAL 0x00000008
> > +#define VSOCK_TRANSPORT_F_LOCAL 0x00000008
> >
> > struct vsock_transport {
> > struct module *module;
> > diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> > index ae5ac5531d96..26c97b33d55a 100644
> > --- a/net/vmw_vsock/af_vsock.c
> > +++ b/net/vmw_vsock/af_vsock.c
> > @@ -139,8 +139,8 @@ struct proto vsock_proto = {
> > static const struct vsock_transport *transport_h2g;
> > /* Transport used for guest->host communication */
> > static const struct vsock_transport *transport_g2h;
> > -/* Transport used for DGRAM communication */
> > -static const struct vsock_transport *transport_dgram;
> > +/* Transport used as a fallback for DGRAM communication */
> > +static const struct vsock_transport *transport_dgram_fallback;
> > /* Transport used for local communication */
> > static const struct vsock_transport *transport_local;
> > static DEFINE_MUTEX(vsock_register_mutex);
> > @@ -439,6 +439,18 @@ vsock_connectible_lookup_transport(unsigned int cid, __u8 flags)
> > return transport;
> > }
> >
> > +static const struct vsock_transport *
> > +vsock_dgram_lookup_transport(unsigned int cid, __u8 flags)
> > +{
> > + const struct vsock_transport *transport;
> > +
> > + transport = vsock_connectible_lookup_transport(cid, flags);
> > + if (transport)
> > + return transport;
> > +
> > + return transport_dgram_fallback;
> > +}
> > +
> > /* Assign a transport to a socket and call the .init transport callback.
> > *
> > * Note: for connection oriented socket this must be called when vsk->remote_addr
> > @@ -475,7 +487,8 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
> >
> > switch (sk->sk_type) {
> > case SOCK_DGRAM:
> > - new_transport = transport_dgram;
> > + new_transport = vsock_dgram_lookup_transport(remote_cid,
> > + remote_flags);
>
> I'm a little bit confused about this:
> 1) Let's create SOCK_DGRAM socket using vsock_create()
> 2) for SOCK_DGRAM it calls 'vsock_assign_transport()' and we go here, remote_cid == -1
> 3) I guess 'vsock_dgram_lookup_transport()' calls logic from 0002 and returns h2g for such remote cid, which is not
> correct I think...
>
> Please correct me if i'm wrong
>
> Thanks, Arseniy
>

As I understand, for the VMCI case, if transport_h2g != NULL, then
transport_h2g == transport_dgram_fallback. In either case,
vsk->transport == transport_dgram_fallback.

For the virtio/vhost case, temporarily vsk->transport == transport_h2g,
but it is unused because vsk->transport->dgram_bind == NULL.

Until SS_CONNECTED is set by connect() and vsk->transport is set
correctly, the send path is barred from using the bad transport.

I guess the recvmsg() path is a little more sketchy, and probably only
works in my test cases because h2g/g2h in the vhost/virtio case have
identical dgram_addr_init() implementations.

I think a cleaner solution is maybe checking in vsock_create() if
dgram_bind is implemented. If it is not, then vsk->transport should be
reset to NULL and a comment added explaining why VMCI requires this.

Then the other calls can begin explicitly checking for vsk->transport ==
NULL.

Actually, on further reflection here, in order for the vsk->transport to
be called in time for ->dgram_addr_init(), it is going to be necessary
to call vsock_assign_transport() in vsock_dgram_bind() anyway.

I think this means that the vsock_assign_transport() call can be removed
from vsock_create() call entirely, and yet VMCI can still dispatch
messages upon bind() calls as needed.

This would then simplify the whole arrangement, if there aren't other
unseen issues.

This sounds like a good approach.

My only question is whether vsock_dgram_bind() is always called for each
dgram socket.

Stefano