info: clarify APICapability and OptionalCapabilities docs

CAPABILITIES.md

@@ -0,0 +1,113 @@
+# Capabilities
+
+A Device reports what it supports so a Controller can avoid sending
+configuration the Device would ignore, and avoid waiting for messages the Device
+will never send. There are **three independent mechanisms**, all in `ZInfoDevice`
+(see [info.proto](./proto/info/info.proto)), with **different semantics**.
+Conflating them is a correctness bug.
+
+| mechanism | field | kind |
+| --- | --- | --- |
+| `APICapability` | `api_capability` | monotonic level |
+| `OptionalCapabilities` | `optional_capabilities` | independent booleans |
+| `Capabilities` | `capabilities` | independent booleans (hardware) |
+
+## APICapability — a level, not a set
+
+`APICapability` covers two kinds of support:
+
+1. **`EdgeDevConfig` fields the Device parses.** Set one on a Device below the
+   level and it is silently ignored.
+2. **Messages or fields the Device sends.** A Controller expecting one from a
+   Device below the level waits for it indefinitely.
+
+**A larger value implies every smaller one.** A Controller MUST test with `>=`:
+
+```text
+if device.api_capability >= API_CAPABILITY_MTU {
+    // safe to set NetworkConfig.mtu and NetworkInstanceConfig.mtu
+}
+```
+
+Equality and set-membership tests are wrong. A Device reporting
+`API_CAPABILITY_SMART_REPORT` (20) also supports 1 through 19, and EVE-OS
+reports a single top value rather than a set.
+
+`API_CAPABILITY_UNSPECIFIED` (0) also covers Devices predating the field, so it
+means "none of the below", not "unknown, try anyway".
+
+### What each value covers
+
+`cfg` = an `EdgeDevConfig` field the Device parses. `rpt` = something the Device
+sends. Rows marked **(?)** are inferred from the enum comment and the commit that
+introduced the value, not stated anywhere authoritative — corrections welcome.
+
+| value | | covers |
+| --- | --- | --- |
+| `RETRY_UPDATE` = 1 | cfg | `BaseOS.retry_update` |
+| `SHUTDOWN` = 2 | cfg | `EdgeDevConfig.shutdown` |
+| `START_DELAY_IN_SECONDS` = 3 | cfg | `AppInstanceConfig.start_delay_in_seconds` |
+| `EDGEVIEW` = 4 | cfg | `EdgeDevConfig.edgeview`, `EdgeViewConfig.token` |
+| `VOLUME_SNAPSHOTS` = 5 | cfg | `AppInstanceConfig.snapshot`, `SnapshotConfig` **(?)** |
+| `NETWORK_INSTANCE_ROUTING` = 6 | cfg | `NetworkInstanceConfig.static_routes`, `.propagate_connected_routes`, and `IPRoute` |
+| `BOOT_MODE` = 7 | cfg | `VmConfig.boot_mode` |
+| `MTU` = 8 | cfg | `NetworkConfig.mtu`, `NetworkInstanceConfig.mtu` |
+| `ADAPTER_USER_LABELS` = 9 | cfg | `SystemAdapter.shared_labels` |
+| `ENFORCED_NET_INTERFACE_ORDER` = 10 | cfg | `VmConfig.enforce_network_interface_order`, and hence `NetworkAdapter.interface_order` and `Adapter.interface_order` |
+| `NTPS_FQDN` = 11 | cfg | NTP servers as FQDN, and more than one — `ipspec.ntp`, `ipspec.more_ntp` **(?)** |
+| `WIN_LIC_PASSTHROUGH` = 12 | cfg | `VmConfig.enable_oem_win_license_key` |
+| `VOLUME_SNAPSHOTS_IMMEDIATE` = 13 | cfg | `SnapshotType.SNAPSHOT_TYPE_IMMEDIATE` **(?)** |
+| `ENCRYPTED_PATCH_ENVELOPE` = 14 | cfg | `EveBinaryArtifact.encrypted_inline`, `.encrypted_volumeref`, `.metadata_cipher_data` |
+| `SINGLE_STACK_IP_NETWORK` = 15 | cfg | `NetworkType.V4Only`, `NetworkType.V6Only` |
+| `CELLULAR_ATTACH_CONFIG` = 16 | cfg | `CellularAccessPoint.attach_apn`, `.attach_ip_type`, `.attach_auth_protocol` |
+| `EDGEVIEW_AUTHENTICATION` = 17 | cfg | EdgeView command authentication; no single field — see [EDGEVIEW-CONTAINER-API.md](https://github.com/lf-edge/eve/blob/master/docs/EDGEVIEW-CONTAINER-API.md) **(?)** |
+| `DISABLE_VTPM` = 18 | cfg | `VmConfig.disable_vtpm` |
+| `LOC_REBOOT_COLLECT_INFO` = 19 | cfg | LOC-initiated reboot and collect-info; `LOCConfig.datastore_collect_info_id` **(?)** |
+| `SMART_REPORT` = 20 | rpt | S.M.A.R.T. data in `ZHardwareHealth.disks`, superseding the deprecated `ZInfoHardware.disks` |
+| `REPORT_TPM_EVENTLOG` = 21 | rpt | `ZAttestQuote.tpm_binary_event_log`, superseding the deprecated `ZAttestQuote.event_log` |
+
+### Adding a value
+
+Appending value *N* asserts that a Device reporting it also supports everything
+below. So:
+
+1. Append at the end. Never insert, never renumber.
+2. Name the field(s) or message(s) it covers, in the enum comment and in the
+   table above.
+3. If the capability depends on build flavor or hardware rather than on version,
+   it belongs in `OptionalCapabilities` instead.
+
+## OptionalCapabilities — independent booleans
+
+Not monotonic; test each separately. These describe properties that vary by
+EVE-OS build flavor rather than by version.
+
+| field | meaning | matters because |
+| --- | --- | --- |
+| `hv_type_kubevirt` | Device runs the Kubevirt hypervisor | required before sending `EdgeNodeCluster`; the KVM flavor cannot join a cluster |
+| `hw_inventory_support` | Device can produce `HardwareInventory` | distinguishes "found no hardware" from "cannot report" when `ZInfoHardware.inventory` is empty |
+| `etcd_snapshot` | Device supports etcd snapshots | EVE-k cluster operations |
+
+An absent boolean means "not supported or not reported". Do not infer support
+from a Device predating the field.
+
+## Capabilities — hardware
+
+Independent booleans describing the platform, not the software:
+
+| field | meaning |
+| --- | --- |
+| `HWAssistedVirtualization` | VMX/SVM on amd64, virtualization extensions on arm64 |
+| `IOVirtualization` | IOMMU / I/O virtualization support |
+
+These bound what a Device can ever run: device passthrough
+(`AppInstanceConfig.adapters`, SR-IOV VFs) will not work without
+`IOVirtualization`, whatever `APICapability` says.
+
+## Checklist for Controller implementers
+
+1. Test `APICapability` with `>=`; treat `0` as "none".
+2. Test each `OptionalCapabilities` and `Capabilities` boolean individually.
+3. Check the gate before sending any field in the table above.
+4. A Device does not report being sent a gated field it does not understand — it
+   just ignores it. That silence is why these gates exist.

README.md

@@ -17,6 +17,7 @@ Documentation is in markdown files in this directory, specifically:
 * The protocol in [APIv2.md](./APIv2.md)
 * Local profile overrides in [PROFILE.md](./PROFILE.md)
 * Object signing in [OBJECT-SIGNING.md](./OBJECT-SIGNING.md)
+* Device capability reporting in [CAPABILITIES.md](./CAPABILITIES.md)
 
 ### Message definitions
 

proto/info/info.proto

@@ -620,7 +620,7 @@ message ZInfoDevice {
   // state of attestation process of eve
   AttestationInfo attestation_info = 50;
 
-  // Capability indicating which new EdgeDevConfig fields which are supported
+  // Level of API support; see APICapability
   APICapability api_capability = 51;
 
   // Reports the remote access status
@@ -644,20 +644,27 @@ message ZInfoDevice {
 
 // OptionalCapabilities indicates any additional capabilities device wants
 // to publish to controller. For example Kubevirt hypervisor is not supported by
-// all eve flavors.
+// all eve flavors. Unlike APICapability these are independent booleans, each
+// tested separately; they vary by build flavor rather than by version.
 message OptionalCapabilities {
   // Virtualization type Kubevirt
   bool hv_type_kubevirt = 1;
+  // Device can report HardwareInventory in ZInfoHardware. When false, an empty
+  // inventory means the device cannot produce one, not that it found no hardware.
   bool hw_inventory_support = 2;
   // Device supports etcd snapshots (e.g. eve-k flavor)
   bool etcd_snapshot = 3;
 }
 
-// Capabilities indicates features in the EdgeDevConfig where there is
-// no easy way to otherwise determine whether or not they are parsed and
-// supported by EVE-OS
-// A larger number indicates all lower numbers are also supported thus
-// this works similar to a version field for the EdgeDevConfig support.
+// APICapability indicates API support a controller cannot otherwise detect:
+// both EdgeDevConfig fields EVE-OS parses, and messages EVE-OS sends - without
+// which a controller would wait indefinitely for e.g. the S.M.A.R.T.
+// information in ZHardwareHealth.
+// A larger number implies all lower numbers, so this works like a version
+// field and a controller must compare with >= rather than test for equality.
+// API_CAPABILITY_UNSPECIFIED also covers EVE-OS versions predating the field.
+// Contrast OptionalCapabilities: independent booleans, varying by build flavor.
+// CAPABILITIES.md lists what each value below covers.
 enum APICapability {
   API_CAPABILITY_UNSPECIFIED = 0;
   API_CAPABILITY_RETRY_UPDATE = 1; // BaseOs.retry_update counter supported
@@ -672,16 +679,17 @@ enum APICapability {
   API_CAPABILITY_ENFORCED_NET_INTERFACE_ORDER = 10; // EVE is able to enforce the user-defined order of application network interfaces
   API_CAPABILITY_NTPS_FQDN = 11; // Allow to set NTP server via FQDN instead of only IP and allow setting several NTP servers
   API_CAPABILITY_WIN_LIC_PASSTHROUGH = 12; // Support for passing through OEM Windows license from host's the ACPI tables to the VM
-  API_CAPABILITY_VOLUME_SNAPSHOTS_IMMEDIATE = 13; // Volume snapshots supported
+  API_CAPABILITY_VOLUME_SNAPSHOTS_IMMEDIATE = 13; // SNAPSHOT_TYPE_IMMEDIATE supported
   API_CAPABILITY_ENCRYPTED_PATCH_ENVELOPE = 14; // Support for Patch Envelope Encryption
   API_CAPABILITY_SINGLE_STACK_IP_NETWORK = 15; // Support for V4Only and V6Only NetworkType
   API_CAPABILITY_CELLULAR_ATTACH_CONFIG = 16; // Support cellular attach configuration
   API_CAPABILITY_EDGEVIEW_AUTHENTICATION = 17; // EdgeView authentication, see https://github.com/lf-edge/eve/blob/master/docs/EDGEVIEW-CONTAINER-API.md
-  API_CAPABILITY_DISABLE_VTPM = 18; // Support for disabling
+  API_CAPABILITY_DISABLE_VTPM = 18; // VmConfig.disable_vtpm supported
   API_CAPABILITY_LOC_REBOOT_COLLECT_INFO = 19; // Support for rebooting and collect-info from LOC
   API_CAPABILITY_SMART_REPORT = 20; // Support for S.M.A.R.T. info on physical storage devices
   API_CAPABILITY_REPORT_TPM_EVENTLOG = 21; // Support for reporting TPM Event Log "as is" without parsing and selectively reporting events
-  // Add new values as new EdgeDevConfig API features are implemented
+  // Append new values as new API features are implemented; never insert or
+  // renumber, since a value asserts that all lower values are also supported.
 }
 
 // Different reasons for a boot/reboot
@@ -1285,7 +1293,8 @@ message ZInfoMsg {
   google.protobuf.Timestamp atTimeStamp = 6;
 }
 
-// Information about hardware capabilities of node
+// Information about hardware capabilities of node. Distinct from APICapability
+// (API support level) and OptionalCapabilities (per-flavor software support).
 message Capabilities {
   // VMX/SVM for amd64 or Arm virtualization extensions for arm64
   bool HWAssistedVirtualization = 2;