On Tue, 2026-05-12 at 17:39 +0200, Stefano Garzarella wrote: > On Tue, May 12, 2026 at 02:32:14PM +0000, Polina Vishneva wrote: > > On Mon, 2026-05-11 at 17:56 +0200, Stefano Garzarella wrote: > > > On Mon, May 11, 2026 at 04:56:10PM +0200, Polina Vishneva wrote: > > > > From: "Denis V. Lunev" <[email protected]> > > > > > > > > When the host initiates an AF_VSOCK connect() to a guest that has not > > > > yet loaded the virtio-vsock transport (i.e. still booting), the caller > > > > blocks for VSOCK_DEFAULT_CONNECT_TIMEOUT (2 seconds), because > > > > vhost_transport_do_send_pkt() silently exits when > > > > vhost_vq_get_backend(vq) returns NULL. > > > > > > Can SO_VM_SOCKETS_CONNECT_TIMEOUT helps on this? > > > > It can, but it might be difficult to find a correct timeout. > > > > And, generally, there's no way to distinguish "the guest hasn't yet > > initialized > > the vq" from "the guest is up and running, but didn't reply to connect() in > > time". That's exactly what this patch is attempting to fix. > > Okay, so please mention this in the commit message, I mean why > SO_VM_SOCKETS_CONNECT_TIMEOUT can't really help.
Will do. > > > > > > > > > > > > > > If the guest doesn't start listening within this timeout, connect() > > > > returns ETIMEDOUT. > > > > > > > > This delay is usually pointless and it doesn't well align with our > > I still don't understand why this is pointless. If an application wants > to wait while sleeping, it can simply increase the timeout long enough > to wait for the VM to start up and use a single `connect()` call, > instead of continuing to try and wasting CPU cycles unnecessarily. > > Hmm, or maybe not, because the driver will definitely be initialized > before the application that wants to listen on that port, so it will > respond that no one is listening, and the `connect()` call will fail > with an `ECONNRESET` error in any case. Right? That's the case indeed. > > If it is the case, is the following line in the commit description > correct? > > If the guest doesn't start listening within this timeout, connect() > returns ETIMEDOUT. > > I mean, also if the application starts to listen within the timeout, I > think the connect() will fail in any case as I pointed out above (this > should be another point in favour of this change) Yes, the commit message should be updated, as well as the code comment. > > > BTW, I think we should explain this more clearly both here and briefly > in the code as well. Definitely. > > > > > behavior at other initialization stages: for example, if a connection is > > > > attempted when the guest driver is already loaded, but when nothing is > > > > listening yet, it returns ECONNRESET immediately without any wait. > > > > > > > > Fix this by checking the RX virtqueue backend in > > > > vhost_transport_send_pkt() before queuing. If the backend is NULL, > > > > return -ECONNREFUSED immediately. > > > > > > > > Signed-off-by: Denis V. Lunev <[email protected]> > > > > Co-developed-by: Polina Vishneva <[email protected]> > > > > Signed-off-by: Polina Vishneva <[email protected]> > > > > --- > > > > drivers/vhost/vsock.c | 10 ++++++++++ > > > > 1 file changed, 10 insertions(+) > > > > > > > > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c > > > > index 1d8ec6bed53e..a3f218292c3a 100644 > > > > --- a/drivers/vhost/vsock.c > > > > +++ b/drivers/vhost/vsock.c > > > > @@ -302,6 +302,16 @@ vhost_transport_send_pkt(struct sk_buff *skb, > > > > struct net *net) > > > > return -ENODEV; > > > > } > > > > > > > > + /* Fast-fail if the guest hasn't enabled the RX vq yet. Reading > > > > + * private_data without vq->mutex is deliberate: even if the > > > > backend becomes > > > > + * NULL right after that check, do_send_pkt() checks it under > > > > the mutex. > > > > + */ > > > > + if (!data_race(READ_ONCE(vsock->vqs[VSOCK_VQ_RX].private_data))) > > > > > > Why not using vhost_vq_get_backend() ? > > > > Because it locks the mutex, which is slow and unacceptable in this hot > > path. > > ehm, sorry, which mutex are you talking about? > > I see just a comment about the mutex to be acquired by the caller, but I > don't see any lock there. Apparently the comment in vhost.h says "Context: Need to call with vq->mutex acquired.", but I guess we're safe to ignore this and use it instead of accessing private_data manually, thanks for pointing this out. > > > > > > > > > Also is READ_ONCE() okay without WRITE_ONCE() where it is set ? > > > > It's racy, but as described here in the comment and in the commit message, > > any possible race outcome is covered by the subsequent checks. > > Okay, so what is the point to call READ_ONCE()? Probably none, it was just there in the initial patch version, and I've decided not to drop it when adding data_race(). Will drop. > > > > > > > { > > > > + rcu_read_unlock(); > > > > + kfree_skb(skb); > > > > + return -ECONNREFUSED; > > > > > > This is a generic send_pkt, is it okay to return ECONNREFUSED in any > > > case? > > > > EHOSTUNREACH would probably be better. > > All the current send_pkt functions only return ENODEV, but it has different > > semantics: they mean that the local device isn't yet ready, while there > > we're > > dealing with the opposite end not being ready. > > In the AF_VSOCK prespective, I see ENODEV like the transport is not > ready, so I think it can eventually fit here too, but also EHOSTUNREACH > is fine, for sure better than ECONNREFUSED. EHOSTUNREACH is indeed a better fit, agreed. > > Thanks, > Stefano > > > > > Best regards, Polina. > > > > > > > > Thanks, > > > Stefano > > > > > > > + } > > > > + > > > > if (virtio_vsock_skb_reply(skb)) > > > > atomic_inc(&vsock->queued_replies); > > > > > > > > > > > > base-commit: 8ab992f815d6736b5c7a6f5fd7bfe7bc106bb3dc > > > > -- > > > > 2.53.0 > > > >
