On 9.09.2024 22:03, Peter Xu wrote:
On Tue, Aug 27, 2024 at 07:54:27PM +0200, Maciej S. Szmigiero wrote:
From: "Maciej S. Szmigiero" <maciej.szmigi...@oracle.com>
load_finish SaveVMHandler allows migration code to poll whether
a device-specific asynchronous device state loading operation had finished.
In order to avoid calling this handler needlessly the device is supposed
to notify the migration code of its possible readiness via a call to
qemu_loadvm_load_finish_ready_broadcast() while holding
qemu_loadvm_load_finish_ready_lock.
Signed-off-by: Maciej S. Szmigiero <maciej.szmigi...@oracle.com>
---
include/migration/register.h | 21 +++++++++++++++
migration/migration.c | 6 +++++
migration/migration.h | 3 +++
migration/savevm.c | 52 ++++++++++++++++++++++++++++++++++++
migration/savevm.h | 4 +++
5 files changed, 86 insertions(+)
diff --git a/include/migration/register.h b/include/migration/register.h
index 4a578f140713..44d8cf5192ae 100644
--- a/include/migration/register.h
+++ b/include/migration/register.h
@@ -278,6 +278,27 @@ typedef struct SaveVMHandlers {
int (*load_state_buffer)(void *opaque, char *data, size_t data_size,
Error **errp);
+ /**
+ * @load_finish
+ *
+ * Poll whether all asynchronous device state loading had finished.
+ * Not called on the load failure path.
+ *
+ * Called while holding the qemu_loadvm_load_finish_ready_lock.
+ *
+ * If this method signals "not ready" then it might not be called
+ * again until qemu_loadvm_load_finish_ready_broadcast() is invoked
+ * while holding qemu_loadvm_load_finish_ready_lock.
[1]
+ *
+ * @opaque: data pointer passed to register_savevm_live()
+ * @is_finished: whether the loading had finished (output parameter)
+ * @errp: pointer to Error*, to store an error if it happens.
+ *
+ * Returns zero to indicate success and negative for error
+ * It's not an error that the loading still hasn't finished.
+ */
+ int (*load_finish)(void *opaque, bool *is_finished, Error **errp);
The load_finish() semantics is a bit weird, especially above [1] on "only
allowed to be called once if ..." and also on the locks.
The point of this remark is that a driver needs to call
qemu_loadvm_load_finish_ready_broadcast() if it wants for the migration
core to call its load_finish handler again.
It looks to me vfio_load_finish() also does the final load of the device.
I wonder whether that final load can be done in the threads,
Here, the problem is that current VFIO VMState has to be loaded from the main
migration thread as it internally calls QEMU core address space modification
methods which explode if called from another thread(s).
then after
everything loaded the device post a semaphore telling the main thread to
continue. See e.g.:
if (migrate_switchover_ack()) {
qemu_loadvm_state_switchover_ack_needed(mis);
}
IIUC, VFIO can register load_complete_ack similarly so it only sem_post()
when all things are loaded? We can then get rid of this slightly awkward
interface. I had a feeling that things can be simplified (e.g., if the
thread will take care of loading the final vmstate then the mutex is also
not needed? etc.).
With just a single call to switchover_ack_needed per VFIO device it would
need to do a blocking wait for the device buffers and config state load
to finish, therefore blocking other VFIO devices from potentially loading
their config state if they are ready to begin this operation earlier.
Thanks,
Maciej
