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[PULL 17/26] virtio-mem: Fail if a memory backend with "prealloc=on" is

From: Juan Quintela
Subject: [PULL 17/26] virtio-mem: Fail if a memory backend with "prealloc=on" is specified
Date: Thu, 2 Feb 2023 17:06:31 +0100

From: David Hildenbrand <david@redhat.com>

"prealloc=on" for the memory backend does not work as expected, as
virtio-mem will simply discard all preallocated memory immediately again.
In the best case, it's an expensive NOP. In the worst case, it's an
unexpected allocation error.

Instead, "prealloc=on" should be specified for the virtio-mem device only,
such that virtio-mem will try preallocating memory before plugging
memory dynamically to the guest. Fail if such a memory backend is

Tested-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>S
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
 hw/virtio/virtio-mem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
index 1ed1f5a4af..02f7b5469a 100644
--- a/hw/virtio/virtio-mem.c
+++ b/hw/virtio/virtio-mem.c
@@ -772,6 +772,12 @@ static void virtio_mem_device_realize(DeviceState *dev, 
Error **errp)
         error_setg(errp, "'%s' property specifies an unsupported memdev",
+    } else if (vmem->memdev->prealloc) {
+        error_setg(errp, "'%s' property specifies a memdev with preallocation"
+                   " enabled: %s. Instead, specify 'prealloc=on' for the"
+                   " virtio-mem device. ", VIRTIO_MEM_MEMDEV_PROP,
+                   object_get_canonical_path_component(OBJECT(vmem->memdev)));
+        return;
     if ((nb_numa_nodes && vmem->node >= nb_numa_nodes) ||

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