[RFC PATCH v3 2/4] virtio_ring: export virtqueue_reinit_vring() for noirq restore

[Sungho Bae <baver.bae@gmail.com>]

From: Sungho Bae <baver.bae@lge.com>

After a device reset in noirq context the existing vrings must be
re-initialized without any memory allocation, because GFP_KERNEL is
not available.

The internal helpers virtqueue_reset_split() and
virtqueue_reset_packed() already reset vring indices and descriptor
state in place.  Add a thin exported wrapper, virtqueue_reinit_vring(),
that dispatches to the appropriate helper based on the ring layout.

This will be used by a subsequent patch that adds noirq system-sleep
PM callbacks for virtio-mmio.

Signed-off-by: Sungho Bae <baver.bae@lge.com>
---
 drivers/virtio/virtio_ring.c | 30 ++++++++++++++++++++++++++++++
 include/linux/virtio_ring.h  |  3 +++
 2 files changed, 33 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index fbca7ce1c6bf..528157b1211d 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -3936,5 +3936,35 @@ void virtqueue_map_sync_single_range_for_device(const struct virtqueue *_vq,
 }
 EXPORT_SYMBOL_GPL(virtqueue_map_sync_single_range_for_device);
 
+/**
+ * virtqueue_reinit_vring - reinitialize vring state without reallocation
+ * @_vq: the virtqueue
+ *
+ * Reset the avail/used indices and descriptor state of an existing
+ * virtqueue so it can be reused after a device reset.  No memory is
+ * allocated or freed, making this safe for use in noirq context.
+ *
+ * The caller must ensure that all in-flight buffers have been completed
+ * or detached before invoking this function.  Calling it with outstanding
+ * descriptors will corrupt the free-list state because num_free is
+ * unconditionally restored to the ring's maximum capacity while the
+ * desc_extra next-chain and free_head reflect only the partial free list
+ * that existed at reset time.
+ */
+void virtqueue_reinit_vring(struct virtqueue *_vq)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	unsigned int num = virtqueue_is_packed(vq) ?
+		vq->packed.vring.num : vq->split.vring.num;
+
+	WARN_ON(vq->vq.num_free != num);
+
+	if (virtqueue_is_packed(vq))
+		virtqueue_reset_packed(vq);
+	else
+		virtqueue_reset_split(vq);
+}
+EXPORT_SYMBOL_GPL(virtqueue_reinit_vring);
+
 MODULE_DESCRIPTION("Virtio ring implementation");
 MODULE_LICENSE("GPL");
diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index c97a12c1cda3..26c7c9d0a151 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -118,6 +118,9 @@ void vring_del_virtqueue(struct virtqueue *vq);
 /* Filter out transport-specific feature bits. */
 void vring_transport_features(struct virtio_device *vdev);
 
+/* Reinitialize a virtqueue without reallocation (safe in noirq context) */
+void virtqueue_reinit_vring(struct virtqueue *_vq);
+
 irqreturn_t vring_interrupt(int irq, void *_vq);
 
 u32 vring_notification_data(struct virtqueue *_vq);
-- 
2.43.0