[Virtio-fs] [PATCH v2 3/4] vhost: Add high-level state save/load functions
Stefan Hajnoczi
stefanha at redhat.com
Tue Jul 18 18:42:12 UTC 2023
On Wed, Jul 12, 2023 at 01:17:01PM +0200, Hanna Czenczek wrote:
> vhost_save_backend_state() and vhost_load_backend_state() can be used by
> vhost front-ends to easily save and load the back-end's state to/from
> the migration stream.
>
> Because we do not know the full state size ahead of time,
> vhost_save_backend_state() simply reads the data in 1 MB chunks, and
> writes each chunk consecutively into the migration stream, prefixed by
> its length. EOF is indicated by a 0-length chunk.
>
> Signed-off-by: Hanna Czenczek <hreitz at redhat.com>
> ---
> include/hw/virtio/vhost.h | 35 +++++++
> hw/virtio/vhost.c | 204 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 239 insertions(+)
>
> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> index d8877496e5..0c282abd4e 100644
> --- a/include/hw/virtio/vhost.h
> +++ b/include/hw/virtio/vhost.h
> @@ -425,4 +425,39 @@ int vhost_set_device_state_fd(struct vhost_dev *dev,
> */
> int vhost_check_device_state(struct vhost_dev *dev, Error **errp);
>
> +/**
> + * vhost_save_backend_state(): High-level function to receive a vhost
> + * back-end's state, and save it in `f`. Uses
I think the GtkDoc syntax is @f instead of `f`.
> + * `vhost_set_device_state_fd()` to get the data from the back-end, and
> + * stores it in consecutive chunks that are each prefixed by their
> + * respective length (be32). The end is marked by a 0-length chunk.
> + *
> + * Must only be called while the device and all its vrings are stopped
> + * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`).
> + *
> + * @dev: The vhost device from which to save the state
> + * @f: Migration stream in which to save the state
> + * @errp: Potential error message
> + *
> + * Returns 0 on success, and -errno otherwise.
> + */
> +int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp);
> +
> +/**
> + * vhost_load_backend_state(): High-level function to load a vhost
> + * back-end's state from `f`, and send it over to the back-end. Reads
> + * the data from `f` in the format used by `vhost_save_state()`, and
> + * uses `vhost_set_device_state_fd()` to transfer it to the back-end.
> + *
> + * Must only be called while the device and all its vrings are stopped
> + * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`).
> + *
> + * @dev: The vhost device to which to send the sate
> + * @f: Migration stream from which to load the state
> + * @errp: Potential error message
> + *
> + * Returns 0 on success, and -errno otherwise.
> + */
> +int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp);
> +
> #endif
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 756b6d55a8..332d49a310 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -2128,3 +2128,207 @@ int vhost_check_device_state(struct vhost_dev *dev, Error **errp)
> "vhost transport does not support migration state transfer");
> return -ENOSYS;
> }
> +
> +int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
> +{
> + /* Maximum chunk size in which to transfer the state */
> + const size_t chunk_size = 1 * 1024 * 1024;
> + void *transfer_buf = NULL;
> + g_autoptr(GError) g_err = NULL;
> + int pipe_fds[2], read_fd = -1, write_fd = -1, reply_fd = -1;
> + int ret;
> +
> + /* [0] for reading (our end), [1] for writing (back-end's end) */
> + if (!g_unix_open_pipe(pipe_fds, FD_CLOEXEC, &g_err)) {
> + error_setg(errp, "Failed to set up state transfer pipe: %s",
> + g_err->message);
> + ret = -EINVAL;
> + goto fail;
> + }
> +
> + read_fd = pipe_fds[0];
> + write_fd = pipe_fds[1];
> +
> + /*
> + * VHOST_TRANSFER_STATE_PHASE_STOPPED means the device must be stopped.
> + * We cannot check dev->suspended, because the back-end may not support
> + * suspending.
> + */
> + assert(!dev->started);
> +
> + /* Transfer ownership of write_fd to the back-end */
> + ret = vhost_set_device_state_fd(dev,
> + VHOST_TRANSFER_STATE_DIRECTION_SAVE,
> + VHOST_TRANSFER_STATE_PHASE_STOPPED,
> + write_fd,
> + &reply_fd,
> + errp);
> + if (ret < 0) {
> + error_prepend(errp, "Failed to initiate state transfer: ");
> + goto fail;
> + }
> +
> + /* If the back-end wishes to use a different pipe, switch over */
> + if (reply_fd >= 0) {
> + close(read_fd);
> + read_fd = reply_fd;
> + }
> +
> + transfer_buf = g_malloc(chunk_size);
> +
> + while (true) {
> + ssize_t read_ret;
> +
> + read_ret = read(read_fd, transfer_buf, chunk_size);
> + if (read_ret < 0) {
Is it necessary to handle -EINTR?
> + ret = -errno;
> + error_setg_errno(errp, -ret, "Failed to receive state");
> + goto fail;
> + }
> +
> + assert(read_ret <= chunk_size);
> + qemu_put_be32(f, read_ret);
> +
> + if (read_ret == 0) {
> + /* EOF */
> + break;
> + }
> +
> + qemu_put_buffer(f, transfer_buf, read_ret);
> + }
> +
> + /*
> + * Back-end will not really care, but be clean and close our end of the pipe
> + * before inquiring the back-end about whether transfer was successful
> + */
> + close(read_fd);
> + read_fd = -1;
> +
> + /* Also, verify that the device is still stopped */
> + assert(!dev->started);
> +
> + ret = vhost_check_device_state(dev, errp);
> + if (ret < 0) {
> + goto fail;
> + }
> +
> + ret = 0;
> +fail:
> + g_free(transfer_buf);
> + if (read_fd >= 0) {
> + close(read_fd);
> + }
> +
> + return ret;
> +}
> +
> +int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
> +{
> + size_t transfer_buf_size = 0;
> + void *transfer_buf = NULL;
> + g_autoptr(GError) g_err = NULL;
> + int pipe_fds[2], read_fd = -1, write_fd = -1, reply_fd = -1;
> + int ret;
> +
> + /* [0] for reading (back-end's end), [1] for writing (our end) */
> + if (!g_unix_open_pipe(pipe_fds, FD_CLOEXEC, &g_err)) {
> + error_setg(errp, "Failed to set up state transfer pipe: %s",
> + g_err->message);
> + ret = -EINVAL;
> + goto fail;
> + }
> +
> + read_fd = pipe_fds[0];
> + write_fd = pipe_fds[1];
> +
> + /*
> + * VHOST_TRANSFER_STATE_PHASE_STOPPED means the device must be stopped.
> + * We cannot check dev->suspended, because the back-end may not support
> + * suspending.
> + */
> + assert(!dev->started);
> +
> + /* Transfer ownership of read_fd to the back-end */
> + ret = vhost_set_device_state_fd(dev,
> + VHOST_TRANSFER_STATE_DIRECTION_LOAD,
> + VHOST_TRANSFER_STATE_PHASE_STOPPED,
> + read_fd,
> + &reply_fd,
> + errp);
> + if (ret < 0) {
> + error_prepend(errp, "Failed to initiate state transfer: ");
> + goto fail;
> + }
> +
> + /* If the back-end wishes to use a different pipe, switch over */
> + if (reply_fd >= 0) {
> + close(write_fd);
> + write_fd = reply_fd;
> + }
> +
> + while (true) {
> + size_t this_chunk_size = qemu_get_be32(f);
> + ssize_t write_ret;
> + const uint8_t *transfer_pointer;
> +
> + if (this_chunk_size == 0) {
> + /* End of state */
> + break;
> + }
> +
> + if (transfer_buf_size < this_chunk_size) {
> + transfer_buf = g_realloc(transfer_buf, this_chunk_size);
> + transfer_buf_size = this_chunk_size;
> + }
> +
> + if (qemu_get_buffer(f, transfer_buf, this_chunk_size) <
> + this_chunk_size)
> + {
> + error_setg(errp, "Failed to read state");
> + ret = -EINVAL;
> + goto fail;
> + }
> +
> + transfer_pointer = transfer_buf;
> + while (this_chunk_size > 0) {
> + write_ret = write(write_fd, transfer_pointer, this_chunk_size);
> + if (write_ret < 0) {
Is it necessary to handle -EINTR?
> + ret = -errno;
> + error_setg_errno(errp, -ret, "Failed to send state");
> + goto fail;
> + } else if (write_ret == 0) {
> + error_setg(errp, "Failed to send state: Connection is closed");
> + ret = -ECONNRESET;
> + goto fail;
> + }
> +
> + assert(write_ret <= this_chunk_size);
> + this_chunk_size -= write_ret;
> + transfer_pointer += write_ret;
> + }
> + }
> +
> + /*
> + * Close our end, thus ending transfer, before inquiring the back-end about
> + * whether transfer was successful
> + */
> + close(write_fd);
> + write_fd = -1;
> +
> + /* Also, verify that the device is still stopped */
> + assert(!dev->started);
> +
> + ret = vhost_check_device_state(dev, errp);
> + if (ret < 0) {
> + goto fail;
> + }
> +
> + ret = 0;
> +fail:
> + g_free(transfer_buf);
> + if (write_fd >= 0) {
> + close(write_fd);
> + }
> +
> + return ret;
> +}
> --
> 2.41.0
>
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