[PATCH v1] vhost-vdpa: Set discarding of RAM broken when initializing the backend

[David Hildenbrand <david@redhat.com>]

Similar to VFIO, vDPA will go ahead an map+pin all guest memory. Memory
that used to be discarded will get re-populated and if we
discard+re-access memory after mapping+pinning, the pages mapped into the
vDPA IOMMU will go out of sync with the actual pages mapped into the user
space page tables.

Set discarding of RAM broken such that:
- virtio-mem and vhost-vdpa run mutually exclusive
- virtio-balloon is inhibited and no memory discards will get issued

In the future, we might be able to support coordinated discarding of RAM
as used by virtio-mem and as planned for VFIO.

Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Cindy Lu <lulu@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---

Note: I was not actually able to reproduce/test as I fail to get the
vdpa_sim/vdpa_sim_net running on upstream Linux (whetever vdpa, vhost_vdpa,
vdpa_sim, vdpa_sim_net modules I probe, and in which order, no vdpa devices
appear under /sys/bus/vdpa/devices/ or /dev/).

---
 hw/virtio/vhost-vdpa.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
index 01d2101d09..86058d4041 100644
--- a/hw/virtio/vhost-vdpa.c
+++ b/hw/virtio/vhost-vdpa.c
@@ -278,6 +278,17 @@ static int vhost_vdpa_init(struct vhost_dev *dev, void *opaque)
     uint64_t features;
     assert(dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_VDPA);
     trace_vhost_vdpa_init(dev, opaque);
+    int ret;
+
+    /*
+     * Similar to VFIO, we end up pinning all guest memory and have to
+     * disable discarding of RAM.
+     */
+    ret = ram_block_discard_disable(true);
+    if (ret) {
+        error_report("Cannot set discarding of RAM broken");
+        return ret;
+    }
 
     v = opaque;
     v->dev = dev;
@@ -302,6 +313,8 @@ static int vhost_vdpa_cleanup(struct vhost_dev *dev)
     memory_listener_unregister(&v->listener);
 
     dev->opaque = NULL;
+    ram_block_discard_disable(false);
+
     return 0;
 }
 
-- 
2.29.2