Re: [PATCH 8/9] vhost-user: add VHOST_USER_SET_DEVICE_STATE_FD command

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 19 Dec 2024 12:13:59 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> +/**
> + * vu_migrate() -- Send/receive passt insternal state to/from QEMU

Magic!

> + * @vdev:	vhost-user device
> + * @events:	epoll events
> + */
> +void vu_migrate(struct vu_dev *vdev, uint32_t events)
> +{
> +	int ret;
> +
> +	/* TODO: collect/set passt internal state
> +         *       and use vdev->device_state_fd to send/receive it
> +         */
> +	debug("vu_migrate fd %d events %x", vdev->device_state_fd, events);
> +	if (events & EPOLLOUT) {

I haven't really reviewed the series yet, but I have a preliminary
question: how does the hypervisor tell us that we're writing too much?

I guess we'll do a short write and we'll need to go back to EPOLLOUT?
There's no minimum chunk size we can write, correct?

> +		debug("Saving backend state");
> +
> +		/* send some stuff */
> +		ret = write(vdev->device_state_fd, "PASST", 6);
> +		/* value to be returned by VHOST_USER_CHECK_DEVICE_STATE */
> +		vdev->device_state_result = ret == -1 ? -1 : 0;
> +		/* Closing the file descriptor signals the end of transfer */
> +		epoll_ctl(vdev->context->epollfd, EPOLL_CTL_DEL,
> +			  vdev->device_state_fd, NULL);
> +		close(vdev->device_state_fd);
> +		vdev->device_state_fd = -1;
> +	} else if (events & EPOLLIN) {

...and similarly here, I guess we'll get a short read?

> +		char buf[6];
> +
> +		debug("Loading backend state");
> +		/* read some stuff */
> +		ret = read(vdev->device_state_fd, buf, sizeof(buf));
> +		/* value to be returned by VHOST_USER_CHECK_DEVICE_STATE */
> +		if (ret != sizeof(buf)) {
> +			vdev->device_state_result = -1;
> +		} else {
> +			ret = strncmp(buf, "PASST", sizeof(buf));
> +			vdev->device_state_result = ret == 0 ? 0 : -1;
> +		}
> +	} else if (events & EPOLLHUP) {
> +		debug("Closing migration channel");
> +
> +		/* The end of file signals the end of the transfer. */
> +		epoll_ctl(vdev->context->epollfd, EPOLL_CTL_DEL,
> +			  vdev->device_state_fd, NULL);
> +		close(vdev->device_state_fd);
> +		vdev->device_state_fd = -1;
> +	}
> +}

-- 
Stefano