hypervisor: kvm: improve logging for triple fault

When a triple fault happens [0], we now get at least a log message. This
helps to better understand the root cause of sudden reboots.

Broader context: We experience reboots caused by triple faults in
edk2 (6 months old as well as recent). They happen so early in the boot
that one doesn't really see them without looking at the VMM log. An
automatic system reset plus reboot often hides these situations - now
they are at least more visible in the log.

PS: Printing the registers to get more debugging help doesn't help, as
the guest already triple-faulted - the CPU state of the root cause
doesn't exist anymore.

[0] https://elixir.bootlin.com/linux/v6.18.6/source/arch/x86/kvm/x86.c#L11123

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

hypervisor/src/kvm/mod.rs

@@ -125,6 +125,7 @@ use kvm_bindings::{KVM_X86_SW_PROTECTED_VM, KVMIO};
 #[cfg(target_arch = "x86_64")]
 use kvm_bindings::{Xsave as xsave2, kvm_xsave2};
 pub use kvm_ioctls::{self, Cap, Kvm, VcpuExit};
+use log::error;
 use thiserror::Error;
 use vfio_ioctls::VfioDeviceFd;
 #[cfg(target_arch = "x86_64")]
@@ -2334,7 +2335,15 @@ impl cpu::Vcpu for KvmVcpu {
                 #[cfg(target_arch = "x86_64")]
                 VcpuExit::IoapicEoi(vector) => Ok(cpu::VmExit::IoapicEoi(vector)),
                 #[cfg(target_arch = "x86_64")]
-                VcpuExit::Shutdown | VcpuExit::Hlt => Ok(cpu::VmExit::Reset),
+                VcpuExit::Shutdown => {
+                    error!("Guest likely triple-faulted");
+                    Ok(cpu::VmExit::Reset)
+                }
+                // Practically unlikely, as KVM emulates the LAPIC and therefore HLT
+                VcpuExit::Hlt => {
+                    error!("Received a HLT exit but KVM should handle this in kernel space");
+                    Ok(cpu::VmExit::Reset)
+                }
 
                 #[cfg(target_arch = "aarch64")]
                 VcpuExit::SystemEvent(event_type, flags) => {