Re: [PATCH v5 2/5] vhost/vsock: suppress EHOSTUNREACH fast-fail during CPR pause

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Jul 20, 2026 at 01:22:38PM +0300, Andrey Drobyshev wrote:
>Earlier commit bb26ed5f3a8b ("vhost/vsock: Refuse the connection
>immediately when guest isn't ready") added a fast-fail in
>vhost_transport_send_pkt().  It rejects every host send with -EHOSTUNREACH
>until the destination calls SET_RUNNING(1).  The fast-fail condition checks
>whether device's backends are dropped, and if they're, the guest is
>considered to be not ready.
>
>However, there might be other reasons for backends to be nulled.  In
>particular, when QEMU is performing CPR (checkpoint-restore) migration,
>device ownership is being RESET and SET again, which leads to backends
>drop and reattach.  If we end up connecting during this window, an
>AF_VSOCK client gets -EHOSTUNREACH, which is wrong.
>
>Add an 'ever_started' flag which is set once in vhost_vsock_start() and is
>never cleared.  The behaviour changes to:
>
>  * When device was never started -> flag is unset -> no listener can
>    exist yet -> fast-fail;
>  * Once the device starts -> flag is set -> we don't fast-fail ->
>    we queue and preserve during any later stop / CPR pause.
>
>The VHOST_RESET_OWNER ioctl is implemented in a following patch, and
>without RESET_OWNER the problem we fix here isn't manifesting - thus
>this patch is a preparation to support RESET_OWNER.
>
>Important caveat: after the first start, a connect during any stopped
>window is queued instead of fast-failed.  That was the behaviour before
>the patch bb26ed5f3a8b, and we're restoring it now.  However we still
>keep the behaviour originally intended by that commit (i.e. fast-fail if
>there's no real listener yet) while fixing the CPR path.
>
>Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>Signed-off-by: Denis V. Lunev <den@openvz.org>
>Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
>Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
>---
> drivers/vhost/vsock.c | 22 ++++++++++++----------
> 1 file changed, 12 insertions(+), 10 deletions(-)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>