On 18.07.23 20:42, Stefan Hajnoczi wrote:
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 <hre...@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`.
OK, I’ll fix the parameter name references!
+ * `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?
Yes, indeed, I do believe so. I’ll wrap this and the write() in
RETRY_ON_EINTR().
Hanna
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