Merge tag 'virtio-mem-for-5.16' of git://github.com/davidhildenbrand/linux

Pull virtio-mem update from David Hildenbrand:
 "Support the VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE feature in virtio-mem,
  now that "accidential" access to logically unplugged memory inside
  added Linux memory blocks is no longer possible, because we:

   - Removed /dev/kmem in commit bbcd53c9 ("drivers/char: remove
     /dev/kmem for good")

   - Disallowed access to virtio-mem device memory via /dev/mem in
     commit 2128f4e2 ("virtio-mem: disallow mapping virtio-mem memory
     via /dev/mem")

   - Sanitized access to virtio-mem device memory via /proc/kcore in
     commit 0daa322b ("fs/proc/kcore: don't read offline sections,
     logically offline pages and hwpoisoned pages")

   - Sanitized access to virtio-mem device memory via /proc/vmcore in
     commit ce281462 ("virtio-mem: kdump mode to sanitize
     /proc/vmcore access")

  The new VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE feature that will be
  required by some hypervisors implementing virtio-mem in the near
  future, so let's support it now that we safely can"

* tag 'virtio-mem-for-5.16' of git://github.com/davidhildenbrand/linux:
  virtio-mem: support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE

drivers/virtio/virtio_mem.c

@@ -2889,6 +2889,7 @@ static unsigned int virtio_mem_features[] = {
 #if defined(CONFIG_NUMA) && defined(CONFIG_ACPI_NUMA)
 	VIRTIO_MEM_F_ACPI_PXM,
 #endif
+	VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE,
 };
 
 static const struct virtio_device_id virtio_mem_id_table[] = {

include/uapi/linux/virtio_mem.h

@@ -68,9 +68,10 @@
  * explicitly triggered (VIRTIO_MEM_REQ_UNPLUG).
  *
  * There are no guarantees what will happen if unplugged memory is
- * read/written. Such memory should, in general, not be touched. E.g.,
- * even writing might succeed, but the values will simply be discarded at
- * random points in time.
+ * read/written. In general, unplugged memory should not be touched, because
+ * the resulting action is undefined. There is one exception: without
+ * VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, unplugged memory inside the usable
+ * region can be read, to simplify creation of memory dumps.
  *
  * It can happen that the device cannot process a request, because it is
  * busy. The device driver has to retry later.
@@ -87,6 +88,8 @@
 
 /* node_id is an ACPI PXM and is valid */
 #define VIRTIO_MEM_F_ACPI_PXM		0
+/* unplugged memory must not be accessed */
+#define VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE	1
 
 
 /* --- virtio-mem: guest -> host requests --- */