memory-device,vhost: Support automatic decision on the number of memslots

We want to support memory devices that can automatically decide how many
memslots they will use. In the worst case, they have to use a single
memslot.

The target use cases are virtio-mem and the hyper-v balloon.

Let's calculate a reasonable limit such a memory device may use, and
instruct the device to make a decision based on that limit. Use a simple
heuristic that considers:
* A memslot soft-limit for all memory devices of 256; also, to not
  consume too many memslots -- which could harm performance.
* Actually still free and unreserved memslots
* The percentage of the remaining device memory region that memory device
  will occupy.

Further, while we properly check before plugging a memory device whether
there still is are free memslots, we have other memslot consumers (such as
boot memory, PCI BARs) that don't perform any checks and might dynamically
consume memslots without any prior reservation. So we might succeed in
plugging a memory device, but once we dynamically map a PCI BAR we would
be in trouble. Doing accounting / reservation / checks for all such
users is problematic (e.g., sometimes we might temporarily split boot
memory into two memslots, triggered by the BIOS).

We use the historic magic memslot number of 509 as orientation to when
supporting 256 memory devices -> memslots (leaving 253 for boot memory and
other devices) has been proven to work reliable. We'll fallback to
suggesting a single memslot if we don't have at least 509 total memslots.

Plugging vhost devices with less than 509 memslots available while we
have memory devices plugged that consume multiple memslots due to
automatic decisions can be problematic. Most configurations might just fail
due to "limit < used + reserved", however, it can also happen that these
memory devices would suddenly consume memslots that would actually be
required by other memslot consumers (boot, PCI BARs) later. Note that this
has always been sketchy with vhost devices that support only a small number
of memslots; but we don't want to make it any worse.So let's keep it simple
and simply reject plugging such vhost devices in such a configuration.

Eventually, all vhost devices that want to be fully compatible with such
memory devices should support a decent number of memslots (>= 509).

Message-ID: <20230926185738.277351-13-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 2bdb4fb231..a546a585da 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1462,6 +1462,19 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
         goto fail;
     }
 
+    limit = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
+    if (limit < MEMORY_DEVICES_SAFE_MAX_MEMSLOTS &&
+        memory_devices_memslot_auto_decision_active()) {
+        error_setg(errp, "some memory device (like virtio-mem)"
+            " decided how many memory slots to use based on the overall"
+            " number of memory slots; this vhost backend would further"
+            " restricts the overall number of memory slots");
+        error_append_hint(errp, "Try plugging this vhost backend before"
+            " plugging such memory devices.\n");
+        r = -EINVAL;
+        goto fail;
+    }
+
     for (i = 0; i < hdev->nvqs; ++i, ++n_initialized_vqs) {
         r = vhost_virtqueue_init(hdev, hdev->vqs + i, hdev->vq_index + i);
         if (r < 0) {
@@ -1548,7 +1561,6 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
      * but we'll need additional information about the reservations.
      */
     reserved = memory_devices_get_reserved_memslots();
-    limit = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
     if (used + reserved > limit) {
         error_setg(errp, "vhost backend memory slots limit (%d) is less"
                    " than current number of used (%d) and reserved (%d)"