[PULL 08/14] vhost-user: make vhost_set_vring_file() synchronous

["Michael S. Tsirkin" <mst@redhat.com>]

From: German Maglione <gmaglione@redhat.com>

QEMU sends all of VHOST_USER_SET_VRING_KICK, _CALL, and _ERR without
setting the NEED_REPLY flag, i.e. by the time the respective
vhost_user_set_vring_*() function returns, it is completely up to chance
whether the back-end has already processed the request and switched over
to the new FD for interrupts.

At least for vhost_user_set_vring_call(), that is a problem: It is
called through vhost_virtqueue_mask(), which is generally used in the
VirtioDeviceClass.guest_notifier_mask() implementation, which is in turn
called by virtio_pci_one_vector_unmask().  The fact that we do not wait
for the back-end to install the FD leads to a race there:

Masking interrupts is implemented by redirecting interrupts to an
internal event FD that is not connected to the guest.  Unmasking then
re-installs the guest-connected IRQ FD, then checks if there are pending
interrupts left on the masked event FD, and if so, issues an interrupt
to the guest.

Because guest_notifier_mask() (through vhost_user_set_vring_call())
doesn't wait for the back-end to switch over to the actual IRQ FD, it's
possible we check for pending interrupts while the back-end is still
using the masked event FD, and then we will lose interrupts that occur
before the back-end finally does switch over.

Fix this by setting NEED_REPLY on those VHOST_USER_SET_VRING_* messages,
so when we get that reply, we know that the back-end is now using the
new FD.

We have a few reports of a virtiofs mount hanging:
- https://gitlab.com/virtio-fs/virtiofsd/-/issues/101
- https://gitlab.com/virtio-fs/virtiofsd/-/issues/133
- https://gitlab.com/virtio-fs/virtiofsd/-/issues/213

This is quite difficult bug to reproduce, even for the reporters.
It only happens on production, every few weeks, and/or on 1 in 300 VMs.
So, we are not 100% sure this fixes that issue. However, we think this
is still a bug, and at least we have one report that claims this fixed
the issue:

https://gitlab.com/virtio-fs/virtiofsd/-/issues/133#note_2743209419

Fixes: 5f6f6664bf24 ("Add vhost-user as a vhost backend.")
Signed-off-by: German Maglione <gmaglione@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20251022162405.318672-1-gmaglione@redhat.com>
---
 hw/virtio/vhost-user.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 4b0fae12ae..63fa9a1b4b 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -1327,8 +1327,11 @@ static int vhost_set_vring_file(struct vhost_dev *dev,
                                 VhostUserRequest request,
                                 struct vhost_vring_file *file)
 {
+    int ret;
     int fds[VHOST_USER_MAX_RAM_SLOTS];
     size_t fd_num = 0;
+    bool reply_supported = virtio_has_feature(dev->protocol_features,
+                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
     VhostUserMsg msg = {
         .hdr.request = request,
         .hdr.flags = VHOST_USER_VERSION,
@@ -1336,13 +1339,32 @@ static int vhost_set_vring_file(struct vhost_dev *dev,
         .hdr.size = sizeof(msg.payload.u64),
     };
 
+    if (reply_supported) {
+        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
+    }
+
     if (file->fd > 0) {
         fds[fd_num++] = file->fd;
     } else {
         msg.payload.u64 |= VHOST_USER_VRING_NOFD_MASK;
     }
 
-    return vhost_user_write(dev, &msg, fds, fd_num);
+    ret = vhost_user_write(dev, &msg, fds, fd_num);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (reply_supported) {
+        /*
+         * wait for the back-end's confirmation that the new FD is active,
+         * otherwise guest_notifier_mask() could check for pending interrupts
+         * while the back-end is still using the masked event FD, losing
+         * interrupts that occur before the back-end installs the FD
+         */
+        return process_message_reply(dev, &msg);
+    }
+
+    return 0;
 }
 
 static int vhost_user_set_vring_kick(struct vhost_dev *dev,
-- 
MST