system: Added an API get_max_vcpu_id to kvm

Added a new API `get_max_vcpu_id` that returns the
`KVM_CAP_MAX_VCPU_ID` if the Extension is present or `get_max_vcpus`.

Also updated the test case for `vm.create_cpu` to use `max_vcpu_id`
obtained using the above API.

Updated coverage score to 91.3 to fix build issue

Signed-off-by: Abhijit Gadgil <gabhijit@iitbombay.org>

coverage_config_x86_64.json

@@ -1,5 +1,5 @@
 {
-  "coverage_score": 91.2,
+  "coverage_score": 91.3,
   "exclude_path": "",
   "crate_features": ""
 }

src/ioctls/system.rs

@@ -227,6 +227,27 @@ impl Kvm {
         }
     }
 
+    /// Gets the Maximum VCPU ID per VM.
+    ///
+    /// See the documentation for `KVM_CAP_MAX_VCPU_ID`
+    /// Returns [get_max_vcpus()](struct.Kvm.html#method.get_max_vcpus) when
+    /// `KVM_CAP_MAX_VCPU_ID` is not implemented
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use kvm_ioctls::Kvm;
+    /// let kvm = Kvm::new().unwrap();
+    /// assert!(kvm.get_max_vcpu_id() > 0);
+    /// ```
+    ///
+    pub fn get_max_vcpu_id(&self) -> usize {
+        match self.check_extension_int(Cap::MaxVcpuId) {
+            0 => self.get_max_vcpus(),
+            x => x as usize,
+        }
+    }
+
     #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
     fn get_cpuid(&self, kind: u64, num_entries: usize) -> Result<CpuId> {
         if num_entries > KVM_MAX_CPUID_ENTRIES {

src/ioctls/vm.rs

@@ -1800,34 +1800,19 @@ mod tests {
     }
 
     #[test]
-    fn create_vcpu_different_cpuids() {
+    fn test_create_vcpu_different_ids() {
         let kvm = Kvm::new().unwrap();
         let vm = kvm.create_vm().unwrap();
 
         // Fails when an arbitrarily large value
         let err = vm.create_vcpu(65537 as u64).err();
         assert_eq!(err.unwrap().errno(), libc::EINVAL);
 
-        // Note: We can request up to KVM_MAX_VCPU_ID if it exists or up to KVM_MAX_VCPUS or
-        // NR_CPUS or 4. This is determined by the appropriate capability being present.
-        // We check near boundry conditions `max_vcpus - 1` should succeed but `max_vcpus` as
-        // determined by the appropriate capability should fail.
-        //
-        // Ref: https://www.kernel.org/doc/html/latest/virt/kvm/api.html#kvm-create-vcpu
-        //
-        let mut max_vcpus = vm.check_extension_int(Cap::MaxVcpuId);
-        if max_vcpus == 0 {
-            max_vcpus = vm.check_extension_int(Cap::MaxVcpus);
-        }
-        if max_vcpus == 0 {
-            max_vcpus = vm.check_extension_int(Cap::NrVcpus);
-        }
-        if max_vcpus == 0 {
-            max_vcpus = 4
-        }
-        let vcpu = vm.create_vcpu((max_vcpus - 1) as u64);
+        // Fails when input `id` = `max_vcpu_id`
+        let max_vcpu_id = kvm.get_max_vcpu_id();
+        let vcpu = vm.create_vcpu((max_vcpu_id - 1) as u64);
         assert!(vcpu.is_ok());
-        let vcpu_err = vm.create_vcpu(max_vcpus as u64).err();
+        let vcpu_err = vm.create_vcpu(max_vcpu_id as u64).err();
         assert_eq!(vcpu_err.unwrap().errno(), libc::EINVAL);
     }