vfio/mlx5: Enforce a single SAVE command at a time

Enforce a single SAVE command at a time.

As the SAVE command is an asynchronous one, we must enforce running only
a single command at a time.

This will preserve ordering between multiple calls and protect from
races on the migration file data structure.

This is a must for the next patches from the series where as part of
PRE_COPY we may have multiple images to be saved and multiple SAVE
commands may be issued from different flows.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20221206083438.37807-4-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/mlx5/cmd.c

@@ -281,6 +281,7 @@ void mlx5vf_mig_file_cleanup_cb(struct work_struct *_work)
 	dma_unmap_sgtable(mdev->device, &migf->table.sgt, DMA_FROM_DEVICE, 0);
 	mlx5_core_dealloc_pd(mdev, async_data->pdn);
 	kvfree(async_data->out);
+	complete(&migf->save_comp);
 	fput(migf->filp);
 }
 
@@ -321,6 +322,10 @@ int mlx5vf_cmd_save_vhca_state(struct mlx5vf_pci_core_device *mvdev,
 		return -ENOTCONN;
 
 	mdev = mvdev->mdev;
+	err = wait_for_completion_interruptible(&migf->save_comp);
+	if (err)
+		return err;
+
 	err = mlx5_core_alloc_pd(mdev, &pdn);
 	if (err)
 		return err;
@@ -371,6 +376,7 @@ int mlx5vf_cmd_save_vhca_state(struct mlx5vf_pci_core_device *mvdev,
 	dma_unmap_sgtable(mdev->device, &migf->table.sgt, DMA_FROM_DEVICE, 0);
 err_dma_map:
 	mlx5_core_dealloc_pd(mdev, pdn);
+	complete(&migf->save_comp);
 	return err;
 }
 

drivers/vfio/pci/mlx5/cmd.h

@@ -37,6 +37,7 @@ struct mlx5_vf_migration_file {
 	unsigned long last_offset;
 	struct mlx5vf_pci_core_device *mvdev;
 	wait_queue_head_t poll_wait;
+	struct completion save_comp;
 	struct mlx5_async_ctx async_ctx;
 	struct mlx5vf_async_data async_data;
 };

drivers/vfio/pci/mlx5/main.c

@@ -245,6 +245,13 @@ mlx5vf_pci_save_device_data(struct mlx5vf_pci_core_device *mvdev)
 	stream_open(migf->filp->f_inode, migf->filp);
 	mutex_init(&migf->lock);
 	init_waitqueue_head(&migf->poll_wait);
+	init_completion(&migf->save_comp);
+	/*
+	 * save_comp is being used as a binary semaphore built from
+	 * a completion. A normal mutex cannot be used because the lock is
+	 * passed between kernel threads and lockdep can't model this.
+	 */
+	complete(&migf->save_comp);
 	mlx5_cmd_init_async_ctx(mvdev->mdev, &migf->async_ctx);
 	INIT_WORK(&migf->async_data.work, mlx5vf_mig_file_cleanup_cb);
 	ret = mlx5vf_cmd_query_vhca_migration_state(mvdev,