vmm: Only return from reset driven I/O once event received

The reset system is asynchronous with an I/O event (PIO or MMIO) for
ACPI/i8042/CMOS triggering a write to the reset_evt event handler. The
VMM thread will pick up this event on the VMM main loop and then trigger
a shutdown in the CpuManager. However since there is some delay between
the CPU threads being marked to be killed (through the
CpuManager::cpus_kill_signalled bool) it is possible for the guest vCPU
that triggered the exit to be re-entered when the vCPU KVM_RUN is called
after the I/O exit is completed.

This is undesirable and in particular the Linux kernel will attempt to
jump to real mode after a CMOS based exit - this is unsupported in
nested KVM on AMD on Azure and will trigger an error in KVM_RUN.

Solve this problem by spinning in the device that has triggered the
reset until the vcpus_kill_signalled boolean has been updated
indicating that the VMM thread has received the event and called
CpuManager::shutdown(). In particular if this bool is set then the vCPU
threads will not re-enter the guest.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

devices/src/acpi.rs

@@ -5,7 +5,9 @@
 
 use super::AcpiNotificationFlags;
 use acpi_tables::{aml, Aml, AmlSink};
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Barrier};
+use std::thread;
 use std::time::Instant;
 use vm_device::interrupt::InterruptSourceGroup;
 use vm_device::BusDevice;
@@ -18,14 +20,20 @@ pub const GED_DEVICE_ACPI_SIZE: usize = 0x1;
 pub struct AcpiShutdownDevice {
     exit_evt: EventFd,
     reset_evt: EventFd,
+    vcpus_kill_signalled: Arc<AtomicBool>,
 }
 
 impl AcpiShutdownDevice {
     /// Constructs a device that will signal the given event when the guest requests it.
-    pub fn new(exit_evt: EventFd, reset_evt: EventFd) -> AcpiShutdownDevice {
+    pub fn new(
+        exit_evt: EventFd,
+        reset_evt: EventFd,
+        vcpus_kill_signalled: Arc<AtomicBool>,
+    ) -> AcpiShutdownDevice {
         AcpiShutdownDevice {
             exit_evt,
             reset_evt,
+            vcpus_kill_signalled,
         }
     }
 }
@@ -43,6 +51,13 @@ impl BusDevice for AcpiShutdownDevice {
             if let Err(e) = self.reset_evt.write(1) {
                 error!("Error triggering ACPI reset event: {}", e);
             }
+            // Spin until we are sure the reset_evt has been handled and that when
+            // we return from the KVM_RUN we will exit rather than re-enter the guest.
+            while !self.vcpus_kill_signalled.load(Ordering::SeqCst) {
+                // This is more effective than thread::yield_now() at
+                // avoiding a priority inversion with the VMM thread
+                thread::sleep(std::time::Duration::from_millis(1));
+            }
         }
         // The ACPI DSDT table specifies the S5 sleep state (shutdown) as value 5
         const S5_SLEEP_VALUE: u8 = 5;
@@ -53,6 +68,13 @@ impl BusDevice for AcpiShutdownDevice {
             if let Err(e) = self.exit_evt.write(1) {
                 error!("Error triggering ACPI shutdown event: {}", e);
             }
+            // Spin until we are sure the reset_evt has been handled and that when
+            // we return from the KVM_RUN we will exit rather than re-enter the guest.
+            while !self.vcpus_kill_signalled.load(Ordering::SeqCst) {
+                // This is more effective than thread::yield_now() at
+                // avoiding a priority inversion with the VMM thread
+                thread::sleep(std::time::Duration::from_millis(1));
+            }
         }
         None
     }