refactor: generify "these features are incompatible" error variants

There's be a lot more things that are incompatible going forward (mostly
related to secret freedom), so instead of adding a ton of error variants
for each pair of incompatible features, let's just have a single one
where we can insert arbitrary features via a string argument.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/vmm/src/resources.rs

@@ -213,7 +213,9 @@ impl VmResources {
                 self.balloon.set_device(balloon);
 
                 if self.machine_config.huge_pages != HugePageConfig::None {
-                    return Err(ResourcesError::BalloonDevice(BalloonConfigError::HugePages));
+                    return Err(ResourcesError::BalloonDevice(
+                        BalloonConfigError::IncompatibleWith("huge pages"),
+                    ));
                 }
             }
 
@@ -255,7 +257,10 @@ impl VmResources {
         }
 
         if self.balloon.get().is_some() && updated.huge_pages != HugePageConfig::None {
-            return Err(MachineConfigError::BalloonAndHugePages);
+            return Err(MachineConfigError::Incompatible(
+                "balloon device",
+                "huge pages",
+            ));
         }
         self.machine_config = updated;
 
@@ -312,7 +317,7 @@ impl VmResources {
         }
 
         if self.machine_config.huge_pages != HugePageConfig::None {
-            return Err(BalloonConfigError::HugePages);
+            return Err(BalloonConfigError::IncompatibleWith("huge pages"));
         }
 
         self.balloon.set(config)
@@ -1394,7 +1399,7 @@ mod tests {
         assert!(
             matches!(
                 err,
-                ResourcesError::BalloonDevice(BalloonConfigError::HugePages)
+                ResourcesError::BalloonDevice(BalloonConfigError::IncompatibleWith("huge pages"))
             ),
             "{:?}",
             err

src/vmm/src/vmm_config/balloon.rs

@@ -28,8 +28,8 @@ pub enum BalloonConfigError {
     CreateFailure(crate::devices::virtio::balloon::BalloonError),
     /// Error updating the balloon device configuration: {0}
     UpdateFailure(std::io::Error),
-    /// Firecracker's huge pages support is incompatible with memory ballooning.
-    HugePages,
+    /// Memory ballooning is incompatible with {0}.
+    IncompatibleWith(&'static str),
 }
 
 /// This struct represents the strongly typed equivalent of the json body

src/vmm/src/vmm_config/machine_config.rs

@@ -27,10 +27,8 @@ pub enum MachineConfigError {
     /// Enabling simultaneous multithreading is not supported on aarch64.
     #[cfg(target_arch = "aarch64")]
     SmtNotSupported,
-    /// Could not determine host kernel version when checking hugetlbfs compatibility
-    KernelVersion,
-    /// Firecracker's huge pages support is incompatible with memory ballooning.
-    BalloonAndHugePages,
+    /// '{0}' and '{1}' are mutually exclusive and cannot be used together.
+    Incompatible(&'static str, &'static str)
 }
 
 /// Describes the possible (huge)page configurations for a microVM's memory.

tests/integration_tests/performance/test_huge_pages.py

@@ -255,7 +255,7 @@ def test_negative_huge_pages_plus_balloon(uvm_plain):
     uvm_plain.basic_config(huge_pages=HugePagesConfig.HUGETLBFS_2MB)
     with pytest.raises(
         RuntimeError,
-        match="Firecracker's huge pages support is incompatible with memory ballooning.",
+        match="Memory ballooning is incompatible with huge pages.",
     ):
         uvm_plain.api.balloon.put(amount_mib=0, deflate_on_oom=False)
 
@@ -264,6 +264,6 @@ def test_negative_huge_pages_plus_balloon(uvm_plain):
     uvm_plain.api.balloon.put(amount_mib=0, deflate_on_oom=False)
     with pytest.raises(
         RuntimeError,
-        match="Machine config error: Firecracker's huge pages support is incompatible with memory ballooning.",
+        match="Machine config error: 'balloon device' and 'huge pages' are mutually exclusive and cannot be used together.",
     ):
         uvm_plain.basic_config(huge_pages=HugePagesConfig.HUGETLBFS_2MB)