[RFC PATCH v4 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 | 51 ++++++++++++++++++++++++++++++++++++
 include/linux/virtio_ring.h  |  3 +++
 2 files changed, 54 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index fbca7ce1c6bf..6631c30cb706 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -506,6 +506,15 @@ static void virtqueue_init(struct vring_virtqueue *vq, u32 num)
 	vq->event_triggered = false;
 	vq->num_added = 0;
 
+	/*
+	 * Keep IN_ORDER state aligned with a freshly initialized/reset queue.
+	 * For packed IN_ORDER, free_head is unused but harmlessly reset.
+	 */
+	if (virtqueue_is_in_order(vq)) {
+		vq->free_head = 0;
+		vq->batch_last.id = UINT_MAX;
+	}
+
 #ifdef DEBUG
 	vq->in_use = false;
 	vq->last_add_time_valid = false;
@@ -3936,5 +3945,47 @@ 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.
+ *
+ * Preconditions for callers:
+ * 1) The vq must be fully quiesced (no concurrent add/get/kick/IRQ callback).
+ * 2) Transport/device side must already have stopped/reset this queue.
+ * 3) All in-flight buffers must already be completed or detached.
+ *
+ * If called with outstanding descriptors, free-list state can be corrupted:
+ * num_free is restored to full capacity while desc_extra next-chain/free_head
+ * may still represent a partially consumed list.
+ */
+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;
+
+	/* All in-flight descriptors must be completed or detached */
+	WARN_ON(vq->vq.num_free != num);
+
+	if (virtqueue_is_packed(vq)) {
+		virtqueue_reset_packed(vq);
+	} else {
+		/*
+		 * Split queue shadow index should match the visible avail
+		 * index when the queue is fully quiesced.
+		 */
+		WARN_ON(vq->split.avail_idx_shadow !=
+			virtio16_to_cpu(vq->vq.vdev,
+					vq->split.vring.avail->idx));
+
+		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