If you need to resend, please add a `vhost-user: ` prefix in the title.
On Wed, Oct 22, 2025 at 03:32:28PM +0200, [email protected] wrote:
From: German Maglione <[email protected]> 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 whether the back-end has already processed the request and
^ There is an extra "whether" here.
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.
IIUC this is related to an issue discovered in virtiofsd, should we put a link to that?
Signed-off-by: German Maglione <[email protected]> Signed-off-by: Hanna Czenczek <[email protected]> --- hw/virtio/vhost-user.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c index 36c9c2e04d..641960293b 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 +1339,26 @@ 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) {
What about adding a comment here that summarizes the excellent description of this commit?
BTW all minor comments, the fix LGTM: Reviewed-by: Stefano Garzarella <[email protected]> Thanks, Stefano
+ return process_message_reply(dev, &msg); + } + + return 0; } static int vhost_user_set_vring_kick(struct vhost_dev *dev, -- 2.49.0
