refactor: use #[from] in VmmError

When a variant wraps another error type, and this error type does not
appear in any other variant, use thiserrors #[from] directive to
implement a `From` impl. This allows eliminating almost all remaining
`map_err`s from `create_vmm_and_vcpus`, as the `?` operator will
automatically call .from() once (although it cannot do it for multiple
layers of wrapping, which is why it was important to change the function
to return VmmError).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/vmm/src/builder.rs

@@ -159,22 +159,22 @@ fn create_vmm_and_vcpus(
     vcpu_count: u8,
     kvm_capabilities: Vec<KvmCapability>,
 ) -> Result<(Vmm, Vec<Vcpu>), VmmError> {
-    let kvm = Kvm::new(kvm_capabilities).map_err(VmmError::Kvm)?;
+    let kvm = Kvm::new(kvm_capabilities)?;
     // Set up Kvm Vm and register memory regions.
     // Build custom CPU config if a custom template is provided.
-    let mut vm = Vm::new(&kvm).map_err(VmmError::Vm)?;
-    kvm.check_memory(&guest_memory).map_err(VmmError::Kvm)?;
-    vm.memory_init(&guest_memory).map_err(VmmError::Vm)?;
+    let mut vm = Vm::new(&kvm)?;
+    kvm.check_memory(&guest_memory)?;
+    vm.memory_init(&guest_memory)?;
 
-    let resource_allocator = ResourceAllocator::new().map_err(VmmError::AllocateResources)?;
+    let resource_allocator = ResourceAllocator::new()?;
 
     // Instantiate the MMIO device manager.
     let mmio_device_manager = MMIODeviceManager::new();
 
     // Instantiate ACPI device manager.
     let acpi_device_manager = ACPIDeviceManager::new();
 
-    let (vcpus, vcpus_exit_evt) = vm.create_vcpus(vcpu_count).map_err(VmmError::Vm)?;
+    let (vcpus, vcpus_exit_evt) = vm.create_vcpus(vcpu_count)?;
 
     #[cfg(target_arch = "x86_64")]
     let pio_device_manager = {

src/vmm/src/lib.rs

@@ -257,9 +257,9 @@ pub enum VmmError {
     /// Cannot spawn Vcpu thread: {0}
     VcpuSpawn(io::Error),
     /// Vm error: {0}
-    Vm(vstate::vm::VmError),
+    Vm(#[from] vstate::vm::VmError),
     /// Kvm error: {0}
-    Kvm(vstate::kvm::KvmError),
+    Kvm(#[from] vstate::kvm::KvmError),
     /// Error thrown by observer object on Vmm initialization: {0}
     VmmObserverInit(vmm_sys_util::errno::Error),
     /// Error thrown by observer object on Vmm teardown: {0}