vmm: Deprecate static CPU templates

Deprecate the `cpu_template` field in the `/machine-config` API, which
is used to set static CPU templates. Users can use custom CPU templates
as an alternative.

To differentiate between explicitly specifying `None` and not specifying
anything, wrap the `cpu_template` of the `MachineConfig` with `Option`.
Thanks to this, it can notify the deprecation of only users using the
field.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

CHANGELOG.md

@@ -42,6 +42,10 @@
 - Changed the dump feature of `cpu-template-helper` tool not to enumerate program
   counter (PC) on ARM because it is determined by the given kernel image and
   it is useless in the custom CPU template context.
+- Deprecated `cpu_template` field in `PUT` and `PATCH` request of `/machine-config`
+  API, which is used to set static CPU templates. Users can use custom CPU templates
+  as an alternative. For more details, please refer here (link to GitHub discussion
+  to be added).
 
 ### Fixed
 

docs/cpu_templates/cpu-templates.md

@@ -34,6 +34,12 @@ Firecracker supports two types of CPU templates:
 - Custom CPU templates - users can create their own CPU templates in json
   format and pass them to Firecracker
 
+> **Note**
+Staitc CPU templates are deprecated starting with v1.5.0. It will be removed in
+accordance with our policy. Even after dropping the support, users can use
+custom CPU templates as an alternative. For more details, please refer here (
+link to the GitHub discussion to be added).
+
 > **Note**
 CPU templates for ARM (both static and custom) require the following patch
 to be available in the host kernel: [Support writable CPU ID registers from userspace](https://lore.kernel.org/kvm/[email protected]/#t).

src/api_server/src/request/machine_configuration.rs

@@ -20,11 +20,25 @@ pub(crate) fn parse_put_machine_config(body: &Body) -> Result<ParsedRequest, Err
         err
     })?;
 
+    // Check for the presence of deprecated `cpu_template` field.
+    let mut deprecation_message = None;
+    if config.cpu_template.is_some() {
+        // `cpu_template` field in request is deprecated.
+        METRICS.deprecated_api.deprecated_http_api_calls.inc();
+        deprecation_message = Some("PUT /machine-config: cpu_template field is deprecated.");
+    }
+
+    // Convert `MachineConfig` to `MachineConfigUpdate`.
     let config_update = MachineConfigUpdate::from(config);
 
-    Ok(ParsedRequest::new_sync(VmmAction::UpdateVmConfiguration(
-        config_update,
-    )))
+    // Construct the `ParsedRequest` object.
+    let mut parsed_req = ParsedRequest::new_sync(VmmAction::UpdateVmConfiguration(config_update));
+    // If `cpu_template` was present, set the deprecation message in `parsing_info`.
+    if let Some(msg) = deprecation_message {
+        parsed_req.parsing_info().append_deprecation_message(msg);
+    }
+
+    Ok(parsed_req)
 }
 
 pub(crate) fn parse_patch_machine_config(body: &Body) -> Result<ParsedRequest, Error> {
@@ -39,9 +53,22 @@ pub(crate) fn parse_patch_machine_config(body: &Body) -> Result<ParsedRequest, E
         return method_to_error(Method::Patch);
     }
 
-    Ok(ParsedRequest::new_sync(VmmAction::UpdateVmConfiguration(
-        config_update,
-    )))
+    // Check for the presence of deprecated `cpu_template` field.
+    let mut deprecation_message = None;
+    if config_update.cpu_template.is_some() {
+        // `cpu_template` field in request is deprecated.
+        METRICS.deprecated_api.deprecated_http_api_calls.inc();
+        deprecation_message = Some("PATCH /machine-config: cpu_template is deprecated.");
+    }
+
+    // Construct the `ParsedRequest` object.
+    let mut parsed_req = ParsedRequest::new_sync(VmmAction::UpdateVmConfiguration(config_update));
+    // If `cpu_template` was present, set the deprecation message in `parsing_info`.
+    if let Some(msg) = deprecation_message {
+        parsed_req.parsing_info().append_deprecation_message(msg);
+    }
+
+    Ok(parsed_req)
 }
 
 #[cfg(test)]
@@ -79,6 +106,24 @@ mod tests {
             "vcpu_count": 8,
             "mem_size_mib": 1024
         }"#;
+        let expected_config = MachineConfigUpdate {
+            vcpu_count: Some(8),
+            mem_size_mib: Some(1024),
+            smt: Some(false),
+            cpu_template: None,
+            track_dirty_pages: Some(false),
+        };
+
+        match vmm_action_from_request(parse_put_machine_config(&Body::new(body)).unwrap()) {
+            VmmAction::UpdateVmConfiguration(config) => assert_eq!(config, expected_config),
+            _ => panic!("Test failed."),
+        }
+
+        let body = r#"{
+            "vcpu_count": 8,
+            "mem_size_mib": 1024,
+            "cpu_template": "None"
+        }"#;
         let expected_config = MachineConfigUpdate {
             vcpu_count: Some(8),
             mem_size_mib: Some(1024),
@@ -102,7 +147,7 @@ mod tests {
             vcpu_count: Some(8),
             mem_size_mib: Some(1024),
             smt: Some(false),
-            cpu_template: Some(StaticCpuTemplate::None),
+            cpu_template: None,
             track_dirty_pages: Some(true),
         };
 
@@ -155,7 +200,7 @@ mod tests {
                 vcpu_count: Some(8),
                 mem_size_mib: Some(1024),
                 smt: Some(true),
-                cpu_template: Some(StaticCpuTemplate::None),
+                cpu_template: None,
                 track_dirty_pages: Some(true),
             };
 

src/vmm/src/cpu_config/templates.rs

@@ -72,6 +72,15 @@ impl From<&Option<CpuTemplateType>> for StaticCpuTemplate {
     }
 }
 
+impl From<&CpuTemplateType> for StaticCpuTemplate {
+    fn from(value: &CpuTemplateType) -> Self {
+        match value {
+            CpuTemplateType::Static(template) => *template,
+            CpuTemplateType::Custom(_) => StaticCpuTemplate::None,
+        }
+    }
+}
+
 impl<'a> TryFrom<&'a [u8]> for CustomCpuTemplate {
     type Error = serde_json::Error;
 

src/vmm/src/vmm_config/machine_config.rs

@@ -40,8 +40,8 @@ pub struct MachineConfig {
     #[serde(default, deserialize_with = "deserialize_smt")]
     pub smt: bool,
     /// A CPU template that it is used to filter the CPU features exposed to the guest.
-    #[serde(default, skip_serializing_if = "StaticCpuTemplate::is_none")]
-    pub cpu_template: StaticCpuTemplate,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub cpu_template: Option<StaticCpuTemplate>,
     /// Enables or disables dirty page tracking. Enabling allows incremental snapshots.
     #[serde(default)]
     pub track_dirty_pages: bool,
@@ -122,7 +122,7 @@ impl From<MachineConfig> for MachineConfigUpdate {
             vcpu_count: Some(cfg.vcpu_count),
             mem_size_mib: Some(cfg.mem_size_mib),
             smt: Some(cfg.smt),
-            cpu_template: Some(cfg.cpu_template),
+            cpu_template: cfg.cpu_template,
             track_dirty_pages: Some(cfg.track_dirty_pages),
         }
     }
@@ -212,7 +212,7 @@ impl From<&VmConfig> for MachineConfig {
             vcpu_count: value.vcpu_count,
             mem_size_mib: value.mem_size_mib,
             smt: value.smt,
-            cpu_template: (&value.cpu_template).into(),
+            cpu_template: value.cpu_template.as_ref().map(|template| template.into()),
             track_dirty_pages: value.track_dirty_pages,
         }
     }