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vhost: Add high-level state save/load functions 

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.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016134243.68248-7-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Hanna Czenczek 2023-10-16 15:42:42 +02:00 committed by Michael S. Tsirkin
parent cda83adc62
commit 4a00d5d7f4
2 changed files with 239 additions and 0 deletions
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204
hw/virtio/vhost.c
View File

@ -2196,3 +2196,207 @@ 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;
g_autofree 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.
* Ideally, it is suspended, but SUSPEND/RESUME currently do not exist for
* vhost-user, so just check that it is stopped at all.
*/
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 = RETRY_ON_EINTR(read(read_fd, transfer_buf, chunk_size));
if (read_ret < 0) {
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:
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;
g_autofree 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.
* Ideally, it is suspended, but SUSPEND/RESUME currently do not exist for
* vhost-user, so just check that it is stopped at all.
*/
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 = RETRY_ON_EINTR(
write(write_fd, transfer_pointer, this_chunk_size)
);
if (write_ret < 0) {
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:
if (write_fd >= 0) {
close(write_fd);
}
return ret;
}

35
include/hw/virtio/vhost.h
View File

@ -429,4 +429,39 @@ 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
* `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