explain replacement component enum

crates/propolis-api-types/src/lib.rs

@@ -51,8 +51,31 @@ pub struct InstanceMetadata {
     pub sled_model: String,
 }
 
-/// Describes an instance spec component that should be replaced during a live
-/// migration.
+/// An instance spec component that should be replaced during a live migration.
+///
+/// When a caller asks Propolis to initialize via live migration, the target VM
+/// inherits the migration source's current instance spec. For the most part,
+/// the target can (and indeed in some cases must) use this spec without
+/// modifying it; this helps Propolis ensure that guest-visible configuration
+/// remains unchanged when a VM migrates. However, there are some components
+/// with no guest-visible state that may need to be reconfigured when a VM
+/// migrates. These include the following:
+///
+/// - Crucible disks: After migrating, the target Propolis presents itself as a
+///   new client of the Crucible downstairs servers backing the VM's disks.
+///   Crucible requires the target to present a newer client generation number
+///   to allow the target to connect. In a full Oxide deployment, these numbers
+///   are managed by the control plane (i.e. it is not safe for Propolis to
+///   manage these values directly--new Crucible volume connection information
+///   must always come from Nexus).
+/// - Virtio network devices: Each virtio NIC in the guest needs to bind to a
+///   named VNIC object on the host. These names can change when a VM migrates
+///   from host to host.
+///
+/// Each component that can be reconfigured this way has a variant in this enum;
+/// components not in the enum can't be reconfigured during migration. This
+/// saves the initialization API from having to reason about requests to replace
+/// a component that can't legally be replaced.
 #[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
 #[serde(deny_unknown_fields, tag = "component", content = "spec")]
 pub enum ReplacementComponent {