[PATCH v5 28/28] virtio_balloon: implement VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED

[Michael S. Tsirkin]

Add VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED (bit 6): when negotiated,
the device guarantees it initializes reported pages (zeros, or
poison_val if PAGE_POISON). The device signals success via the
used length of each reporting_vq element.

Gate host_zeroes_pages on both the feature bit and the actual
page content: when PAGE_POISON is negotiated with poison_val != 0,
the device fills with poison bytes, not zeros.

Clear the feature in validate() if REPORTING is not present or if running in a
confidential computing environment (untrusted host).

Renumber DEVICE_INIT_ON_INFLATE from bit 6 to bit 7 to make room.

See the virtio spec change:
https://github.com/oasis-tcs/virtio-spec/issues/244

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh

virtio_balloon: skip zeroing for host-zeroed reported pages

Check per-page used length returned by the device to determine
which reported pages were zeroed. If used_len matches the page
size, the device successfully initialized the page (e.g. via
MADV_DONTNEED), and we set the corresponding zeroed_bitmap bit.

This requires no feature negotiation: existing devices return
used_len=0 (the conservative "not zeroed" case), while updated
devices return the page size on successful discard.

host_zeroes_pages is set unconditionally so the page_reporting
drain path checks the bitmap and marks matching pages as PG_zeroed
in the buddy allocator.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 drivers/virtio/virtio_balloon.c     | 30 ++++++++++++++++++++++++++---
 include/uapi/linux/virtio_balloon.h |  3 ++-
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 1eb9a6376038..f1ad842eb3d6 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -218,6 +218,8 @@ static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_i
 	struct virtqueue *vq = vb->reporting_vq;
 	unsigned int i, err;
 
+	bitmap_zero(pr_dev_info->zeroed_bitmap, nents);
+
 	/* We should always be able to add these buffers to an empty queue. */
 	for (i = 0; i < nents; i++) {
 		struct scatterlist one;
@@ -237,10 +239,14 @@ static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_i
 
 		/* When host has read buffer, this completes via balloon_ack */
 		for (i = 0; i < nents; i++) {
-			unsigned int unused;
+			struct scatterlist *entry;
+			unsigned int used_len;
 
 			wait_event(vb->acked,
-				   virtqueue_get_buf(vq, &unused));
+				   (entry = virtqueue_get_buf(vq, &used_len)));
+			if (used_len == entry->length)
+				set_bit(entry - sg,
+					pr_dev_info->zeroed_bitmap);
 		}
 	}
 
@@ -1118,8 +1124,16 @@ static int virtballoon_probe(struct virtio_device *vdev)
 #endif
 
 		vb->pr_dev_info.capacity = capacity;
+		/*
+		 * With PAGE_POISON, device fills with poison_val not
+		 * zeros; only treat as zeroed when poison_val is 0.
+		 */
 		vb->pr_dev_info.host_zeroes_pages =
-			!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT);
+			virtio_has_feature(vdev,
+					   VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED) &&
+			(!virtio_has_feature(vdev,
+					    VIRTIO_BALLOON_F_PAGE_POISON) ||
+			 want_init_on_free());
 		err = page_reporting_register(&vb->pr_dev_info);
 		if (err)
 			goto out_unregister_oom;
@@ -1245,9 +1259,18 @@ static int virtballoon_validate(struct virtio_device *vdev)
 	else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON))
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
 
+	if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_REPORTING))
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED);
+
+	/* Device fills with poison_val, not zeros; disable zeroed hint */
 	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON) &&
 	    !want_init_on_free())
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE);
+
+	if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) {
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED);
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE);
+	}
 	__virtio_clear_bit(vdev, VIRTIO_F_ACCESS_PLATFORM);
 	return 0;
 }
@@ -1259,6 +1282,7 @@ static unsigned int features[] = {
 	VIRTIO_BALLOON_F_FREE_PAGE_HINT,
 	VIRTIO_BALLOON_F_PAGE_POISON,
 	VIRTIO_BALLOON_F_REPORTING,
+	VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED,
 	VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE,
 };
 
diff --git a/include/uapi/linux/virtio_balloon.h b/include/uapi/linux/virtio_balloon.h
index d129736cc3a8..cbaf18e0b17c 100644
--- a/include/uapi/linux/virtio_balloon.h
+++ b/include/uapi/linux/virtio_balloon.h
@@ -37,7 +37,8 @@
 #define VIRTIO_BALLOON_F_FREE_PAGE_HINT	3 /* VQ to report free pages */
 #define VIRTIO_BALLOON_F_PAGE_POISON	4 /* Guest is using page poisoning */
 #define VIRTIO_BALLOON_F_REPORTING	5 /* Page reporting virtqueue */
-#define VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE	6 /* Device initializes pages on inflate */
+#define VIRTIO_BALLOON_F_DEVICE_INIT_REPORTED	6 /* Device initializes reported pages */
+#define VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE	7 /* Device initializes pages on inflate */
 
 /* Size of a PFN in the balloon interface. */
 #define VIRTIO_BALLOON_PFN_SHIFT 12
-- 
MST