Change hot-plug API endpoint

Change from `add` to `target`. The user now supplies the number of vCPUs
they would like to reach, rather than the number of vCPUs to add to the
system. Obviously to make this work, hot-unplug functionality is needed
as well.

Examples:
- Currently 1 vCPU, would like 16 vCPUs: target = 16
- Currently 32 vCPUs, would like 2 vCPUs: target = 2

Signed-off-by: James Curtis <[email protected]>

Author: JamesC1305

src/vmm/src/lib.rs

@@ -615,31 +615,23 @@ impl Vmm {
         config: HotplugVcpuConfig,
     ) -> Result<MachineConfigUpdate, HotplugVcpuError> {
         use crate::logger::IncMetric;
-        if config.add < 1 {
-            return Err(HotplugVcpuError::VcpuCountTooLow);
-        } else if self
-            .vcpus_handles
-            .len()
-            .checked_add(config.add.into())
-            .ok_or(HotplugVcpuError::VcpuCountTooHigh)?
-            > MAX_SUPPORTED_VCPUS.into()
-        {
+        if config.target > MAX_SUPPORTED_VCPUS.into() {
             return Err(HotplugVcpuError::VcpuCountTooHigh);
         }
 
         if let Some(kvm_config) = self.vcpu_config.as_mut() {
-            kvm_config.vcpu_count += config.add;
+            kvm_config.vcpu_count = config.target;
         }
         // Create and start new vcpus
-        let mut vcpus = Vec::with_capacity(config.add.into());
+        let mut vcpus = Vec::with_capacity(config.target.into());
 
         #[allow(clippy::cast_possible_truncation)]
         let start_idx = self.vcpus_handles.len().try_into().unwrap();
         if let Some(devices::BusDevice::CpuContainer(cont)) =
             self.get_bus_device(DeviceType::CpuContainer, "CpuContainer")
         {
             let mut locked_container = cont.lock().expect("Poisoned lock");
-            for cpu_idx in start_idx..(start_idx + config.add) {
+            for cpu_idx in start_idx..config.target {
                 let exit_evt = self
                     .vcpus_exit_evt
                     .try_clone()
@@ -666,7 +658,10 @@ impl Vmm {
         .map_err(HotplugVcpuError::VcpuStart)?;
 
         #[allow(clippy::cast_lossless)]
-        METRICS.hotplug.vcpus_added.add(config.add.into());
+        METRICS
+            .hotplug
+            .vcpus_added
+            .add(self.vcpus_handles.len() as u64 - config.target as u64);
 
         // Update VM config to reflect new CPUs added
         #[allow(clippy::cast_possible_truncation)]
@@ -686,6 +681,7 @@ impl Vmm {
         Ok(new_machine_config)
     }
 
+    /// Removes vCPUs from VMM.
     /// Retrieves the KVM dirty bitmap for each of the guest's memory regions.
     pub fn reset_dirty_bitmap(&self) {
         self.guest_memory

src/vmm/src/vmm_config/hotplug.rs

@@ -50,6 +50,6 @@ pub enum HotplugVcpuError {
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
 #[serde(deny_unknown_fields)]
 pub struct HotplugVcpuConfig {
-    /// Number of vcpus to start.
-    pub add: u8,
+    /// Number of vcpus after operation.
+    pub target: u8,
 }