hypervisor: kvm: check for KVM_SET_MSRS failures

The KVM docs say

  If setting an MSR fails, ..., it stops processing the MSR list and
  returns the number of MSRs that have been set successfully.

So, a return value of zero really means the operation completely failed.

Change-Id: Ic98cdb02513ff86e22fb6b048e7ceb2d6b7e554a
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4500732
Reviewed-by: Elie Kheirallah <khei@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Commit-Queue: Frederick Mayle <fmayle@google.com>
Reviewed-by: Noah Gold <nkgold@google.com>

hypervisor/src/kvm/x86_64.rs

@@ -903,11 +903,25 @@ impl VcpuX86_64 for KvmVcpu {
             ioctl_with_ref(self, KVM_SET_MSRS(), &msrs[0])
         };
         // KVM_SET_MSRS actually returns the number of msr entries written.
-        if ret >= 0 {
-            Ok(())
-        } else {
-            errno_result()
+        if ret < 0 {
+            return errno_result();
         }
+        let num_set = ret as usize;
+        if num_set != vec.len() {
+            if let Some(register) = vec.get(num_set) {
+                error!(
+                    "failed to set MSR {:#x?} to {:#x?}",
+                    register.id, register.value
+                );
+            } else {
+                error!(
+                    "unexpected KVM_SET_MSRS return value {num_set} (nmsrs={})",
+                    vec.len()
+                );
+            }
+            return Err(base::Error::new(libc::EPERM));
+        }
+        Ok(())
     }
 
     fn set_cpuid(&self, cpuid: &CpuId) -> Result<()> {

hypervisor/tests/kvm/x86_64.rs

@@ -408,6 +408,20 @@ fn set_msrs() {
     assert_eq!(msrs[0].value, 42);
 }
 
+#[test]
+fn set_msrs_unsupported() {
+    let kvm = Kvm::new().unwrap();
+    let gm = GuestMemory::new(&[(GuestAddress(0), 0x10000)]).unwrap();
+    let vm = KvmVm::new(&kvm, gm, Default::default()).unwrap();
+    let vcpu = vm.create_vcpu(0).unwrap();
+
+    let msrs = vec![Register {
+        id: u32::MAX,
+        value: u64::MAX,
+    }];
+    assert_eq!(vcpu.set_msrs(&msrs), Err(base::Error::new(libc::EPERM)));
+}
+
 #[test]
 fn get_hyperv_cpuid() {
     let kvm = Kvm::new().unwrap();