[snapshot]: Kick pio serial after restore

After resuming from snapshot, the console is unresponsive. This
issue is caused by the fact that we are not saving the serial
console state. After restore Received Data Available interrupt
was not set thus driver did not accept any input.

Signed-off-by: AlexandruCihodaru <[email protected]>
Suggested-by: Laura Loghin <[email protected]>
Signed-off-by: AlexandruCihodaru <[email protected]>

Author: AlexandruCihodaru

src/devices/src/legacy/serial.rs

@@ -29,6 +29,12 @@ use utils::epoll::EventSet;
 /// Trait that composes the `std::io::Read` and `std::os::unix::io::AsRawFd` traits.
 pub trait ReadableFd: io::Read + AsRawFd {}
 
+// Received Data Available interrupt - for letting the driver know that
+// there is some pending data to be processed.
+pub const IER_RDA_BIT: u8 = 0b0000_0001;
+// Received Data Available interrupt offset
+pub const IER_RDA_OFFSET: u8 = 1;
+
 #[derive(Debug)]
 pub enum RawIOError {
     Serial(SerialError<io::Error>),

src/vmm/src/builder.rs

@@ -528,6 +528,8 @@ pub fn build_microvm_from_snapshot(
         MMIODeviceManager::restore(mmio_ctor_args, &microvm_state.device_states)
             .map_err(MicrovmStateError::RestoreDevices)
             .map_err(RestoreMicrovmState)?;
+    vmm.emulate_serial_init()
+        .map_err(StartMicrovmError::Internal)?;
 
     // Move vcpus to their own threads and start their state machine in the 'Paused' state.
     vmm.start_vcpus(

src/vmm/src/lib.rs

@@ -52,6 +52,7 @@ use crate::vstate::{
     vm::Vm,
 };
 use arch::DeviceType;
+use devices::legacy::serial::{IER_RDA_BIT, IER_RDA_OFFSET};
 use devices::virtio::balloon::Error as BalloonError;
 use devices::virtio::{
     Balloon, BalloonConfig, BalloonStats, Block, MmioTransport, Net, BALLOON_DEV_ID, TYPE_BALLOON,
@@ -351,6 +352,28 @@ impl Vmm {
         &self.guest_memory
     }
 
+    /// Sets RDA bit in serial console
+    pub fn emulate_serial_init(&self) -> Result<()> {
+        #[cfg(target_arch = "x86_64")]
+        let mut serial = self
+            .pio_device_manager
+            .stdio_serial
+            .lock()
+            .expect("Poisoned lock");
+
+        // When restoring from a previously saved state, there is no serial
+        // driver initialization, therefore the RDA (Received Data Available)
+        // interrupt is not enabled. Because of that, the driver won't get
+        // notified of any bytes that we send to the guest. The clean solution
+        // would be to save the whole serial device state when we do the vm
+        // serialization. For now we set that bit manually
+        serial
+            .serial
+            .write(IER_RDA_OFFSET, IER_RDA_BIT)
+            .map_err(|_| Error::Serial(std::io::Error::last_os_error()))?;
+        Ok(())
+    }
+
     /// Injects CTRL+ALT+DEL keystroke combo in the i8042 device.
     #[cfg(target_arch = "x86_64")]
     pub fn send_ctrl_alt_del(&mut self) -> Result<()> {