Avoid starting APs that are not associated with any VM

Author: ALSchwalm

mythril/src/kmain.rs

@@ -270,13 +270,19 @@ unsafe fn kmain(mut boot_info: BootInfo) -> ! {
 
     debug!("AP_STARTUP address: 0x{:x}", AP_STARTUP_ADDR);
 
-    //TODO(alschwalm): Only the cores that are actually associated with a VM
-    // should be started
     for (idx, apic_id) in apic_ids.into_iter().enumerate() {
         if apic_id == local_apic.id() {
             continue;
         }
 
+        let core_id = percore::CoreId::from(idx as u32);
+
+        // Do not setup cores that are not allocated to any guest
+        if !vm::is_assigned_core_id(core_id) {
+            debug!("Not starting core ID '{}' because it is not assigned to a guest", core_id);
+            continue;
+        }
+
         // Allocate a stack for the AP
         let stack = vec![0u8; 100 * 1024];
 
@@ -289,7 +295,7 @@ unsafe fn kmain(mut boot_info: BootInfo) -> ! {
         core::ptr::write_volatile(&mut AP_STACK_ADDR as *mut u64, stack_bottom);
 
         // Map the APIC ids to a sequential list and pass it to the AP
-        core::ptr::write_volatile(&mut AP_IDX as *mut u64, idx as u64);
+        core::ptr::write_volatile(&mut AP_IDX as *mut u64, core_id.raw as u64);
 
         // mfence to ensure that the APs see the new stack address
         core::sync::atomic::fence(core::sync::atomic::Ordering::SeqCst);

mythril/src/vcpu.rs

@@ -34,11 +34,16 @@ pub fn mp_entry_point() -> ! {
             .expect("Failed to initialize per-core timer wheel");
     }
 
+    let core_id = percore::read_core_id();
     let vm = unsafe {
-        let core_id = percore::read_core_id();
         vm::get_vm_for_core_id(core_id)
             .expect(&format!("Failed to find VM associated with {}", core_id))
     };
+    debug!(
+        "Starting core ID '{}' as part of vm id '{}'",
+        core_id,
+        vm.read().id
+    );
 
     let vcpu = VCpu::new(vm).expect("Failed to create vcpu");
     vcpu.launch().expect("Failed to launch vm")

mythril/src/vm.rs

@@ -71,6 +71,10 @@ pub fn max_vm_id() -> u32 {
     VIRTUAL_MACHINES.count()
 }
 
+pub fn is_assigned_core_id(core_id: percore::CoreId) -> bool {
+    VIRTUAL_MACHINES.is_assigned_core_id(core_id)
+}
+
 const MAX_PENDING_MSG: usize = 100;
 
 pub enum VirtualMachineMsg {
@@ -113,6 +117,10 @@ impl VirtualMachines {
         self.machine_count
     }
 
+    pub fn is_assigned_core_id(&self, core_id: percore::CoreId) -> bool {
+        self.map.contains_key(&core_id)
+    }
+
     pub fn get_by_core_id(
         &self,
         core_id: percore::CoreId,