virtio-mem: Support "x-ignore-shared" migration

To achieve desired "x-ignore-shared" functionality, we should not
discard all RAM when realizing the device and not mess with
preallocation/postcopy when loading device state. In essence, we should
not touch RAM content.

As "x-ignore-shared" gets set after realizing the device, we cannot
rely on that. Let's simply skip discarding of RAM on incoming migration.
Note that virtio_mem_post_load() will call
virtio_mem_restore_unplugged() -- unless "x-ignore-shared" is set. So
once migration finished we'll have a consistent state.

The initial system reset will also not discard any RAM, because
virtio_mem_unplug_all() will not call virtio_mem_unplug_all() when no
memory is plugged (which is the case before loading the device state).

Note that something like VM templating -- see commit b17fbbe55cba
("migration: allow private destination ram with x-ignore-shared") -- is
currently incompatible with virtio-mem and ram_block_discard_range() will
warn in case a private file mapping is supplied by virtio-mem.

For VM templating with virtio-mem, it makes more sense to either
(a) Create the template without the virtio-mem device and hotplug a
    virtio-mem device to the new VM instances using proper own memory
    backend.
(b) Use a virtio-mem device that doesn't provide any memory in the
    template (requested-size=0) and use private anonymous memory.

Message-ID: <20230706075612.67404-5-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 38 insertions, 9 deletions

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index a922c21380..3f41e00e74 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -18,6 +18,7 @@
 #include "sysemu/numa.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/reset.h"
+#include "sysemu/runstate.h"
 #include "hw/virtio/virtio.h"
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/virtio-mem.h"
@@ -901,11 +902,23 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
         return;
     }
 
-    ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
-    if (ret) {
-        error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
-        ram_block_coordinated_discard_require(false);
-        return;
+    /*
+     * We don't know at this point whether shared RAM is migrated using
+     * QEMU or migrated using the file content. "x-ignore-shared" will be
+     * configured after realizing the device. So in case we have an
+     * incoming migration, simply always skip the discard step.
+     *
+     * Otherwise, make sure that we start with a clean slate: either the
+     * memory backend might get reused or the shared file might still have
+     * memory allocated.
+     */
+    if (!runstate_check(RUN_STATE_INMIGRATE)) {
+        ret = ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb));
+        if (ret) {
+            error_setg_errno(errp, -ret, "Unexpected error discarding RAM");
+            ram_block_coordinated_discard_require(false);
+            return;
+        }
     }
 
     virtio_mem_resize_usable_region(vmem, vmem->requested_size, true);
@@ -977,10 +990,6 @@ static int virtio_mem_post_load(void *opaque, int version_id)
     RamDiscardListener *rdl;
     int ret;
 
-    if (vmem->prealloc && !vmem->early_migration) {
-        warn_report("Proper preallocation with migration requires a newer QEMU machine");
-    }
-
     /*
      * We started out with all memory discarded and our memory region is mapped
      * into an address space. Replay, now that we updated the bitmap.
@@ -993,6 +1002,18 @@ static int virtio_mem_post_load(void *opaque, int version_id)
         }
     }
 
+    /*
+     * If shared RAM is migrated using the file content and not using QEMU,
+     * don't mess with preallocation and postcopy.
+     */
+    if (migrate_ram_is_ignored(vmem->memdev->mr.ram_block)) {
+        return 0;
+    }
+
+    if (vmem->prealloc && !vmem->early_migration) {
+        warn_report("Proper preallocation with migration requires a newer QEMU machine");
+    }
+
     if (migration_in_incoming_postcopy()) {
         return 0;
     }
@@ -1026,6 +1047,14 @@ static int virtio_mem_post_load_early(void *opaque, int version_id)
     }
 
     /*
+     * If shared RAM is migrated using the file content and not using QEMU,
+     * don't mess with preallocation and postcopy.
+     */
+    if (migrate_ram_is_ignored(rb)) {
+        return 0;
+    }
+
+    /*
      * We restored the bitmap and verified that the basic properties
      * match on source and destination, so we can go ahead and preallocate
      * memory for all plugged memory blocks, before actual RAM migration starts