[PATCH RFC v2 15/18] virtio_balloon: add host_zeroes_pages module parameter

["Michael S. Tsirkin" <mst@redhat.com>]

Add a module parameter to opt in to the pre-zeroed page
optimization.  A proper virtio feature flag is needed before
this can be merged.

  insmod virtio_balloon.ko host_zeroes_pages=1

When host_zeroes_pages is set, callers skip folio_zero_user() for
pages that are known to have been zeroed by the host.  This is safe
on cache-aliasing architectures because the hypervisor invalidates
guest cache lines when reclaiming page backing (e.g. MADV_DONTNEED),
so no stale cache state exists when the guest re-maps the page.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh
---
 drivers/virtio/virtio_balloon.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index d1fbc8fe8470..2e524bf6f934 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -19,6 +19,11 @@
 #include <linux/mm.h>
 #include <linux/page_reporting.h>
 
+static bool host_zeroes_pages;
+module_param(host_zeroes_pages, bool, 0444);
+MODULE_PARM_DESC(host_zeroes_pages,
+		 "Host zeroes reported pages, skip guest re-zeroing");
+
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by
  * multiple balloon pages.  All memory counters in this driver are in balloon
@@ -1039,6 +1044,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		vb->pr_dev_info.order = 5;
 #endif
 
+		vb->pr_dev_info.host_zeroes_pages = host_zeroes_pages;
 		err = page_reporting_register(&vb->pr_dev_info);
 		if (err)
 			goto out_unregister_oom;
-- 
MST