[RFC PATCH 07/12] virtio: locate the device memory buffer after feature negotiation

[Alexander Graf <graf@amazon.com>]

In preparation to support VIRTIO_F_DMB, locate a device's Device Memory
Buffer once feature negotiation is complete. The shared memory id that
locates the region may only be read after the device has confirmed
FEATURES_OK, and finalize_features() runs before that.

virtio_features_ok() is the one place in the core that has just read
FEATURES_OK back, so locate the region from there, and release it from
virtio_dev_remove() and from the error paths of probe and restore.
Restore and reset completion reach it too, where virtio_dmb_init() keeps
the state of a device reporting the region it had.

Suspend and reset need no handling of their own. Neither
virtio_device_freeze() nor virtio_reset_device() deletes a virtqueue,
and a live virtqueue holds addresses inside the region, so we keep it
across both.

Link: https://lore.kernel.org/virtio-comment/20260804161202.38619-1-graf@amazon.com/
Assisted-by: Kiro:claude-opus-5 checkpatch sparse
Signed-off-by: Alexander Graf <graf@amazon.com>
---
 drivers/virtio/virtio.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 299fa83be1d5..6f112593566c 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -8,6 +8,8 @@
 #include <linux/of.h>
 #include <uapi/linux/virtio_ids.h>
 
+#include "virtio_dmb.h"
+
 /* Unique numbering for virtio devices. */
 static DEFINE_IDA(virtio_index_ida);
 
@@ -231,7 +233,14 @@ static int virtio_features_ok(struct virtio_device *dev)
 			status);
 		return -ENODEV;
 	}
-	return 0;
+
+	/*
+	 * Negotiation is complete, so a Device Memory Buffer may now be
+	 * located.  Reached from probe, from resume and from reset
+	 * completion, all of which have to end with the state matching what
+	 * the device reports now.
+	 */
+	return virtio_dmb_init(dev);
 }
 
 /**
@@ -361,6 +370,7 @@ static int virtio_dev_probe(struct device *_d)
 
 err:
 	virtio_add_status(dev, VIRTIO_CONFIG_S_FAILED);
+	virtio_dmb_destroy(dev);
 	return err;
 
 }
@@ -377,6 +387,8 @@ static void virtio_dev_remove(struct device *_d)
 	/* Driver should have reset device. */
 	WARN_ON_ONCE(dev->config->get_status(dev));
 
+	virtio_dmb_destroy(dev);
+
 	/* Acknowledge the device's existence again. */
 	virtio_add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);
 
@@ -650,6 +662,7 @@ static int virtio_device_restore_priv(struct virtio_device *dev, bool restore)
 
 err:
 	virtio_add_status(dev, VIRTIO_CONFIG_S_FAILED);
+	virtio_dmb_destroy(dev);
 	return ret;
 }