vmm: Add new "null" serial/console output mode

Poor performance was observed when booting kernels with "console=ttyS0"
and the serial port disabled.

This change introduces a "null" console output mode and makes it the
default for the serial console. In this case the serial port
is advertised as per other output modes but there is no input and any
output is dropped.

Fixes: #163

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/config.rs

@@ -350,6 +350,7 @@ pub enum ConsoleOutputMode {
     Off,
     Tty,
     File,
+    Null,
 }
 
 impl ConsoleOutputMode {
@@ -383,6 +384,11 @@ impl<'a> ConsoleConfig<'a> {
                 mode: ConsoleOutputMode::File,
                 file: Some(Path::new(&param[5..])),
             })
+        } else if param.starts_with("null") {
+            Ok(Self {
+                mode: ConsoleOutputMode::Null,
+                file: None,
+            })
         } else {
             Err(Error::ParseConsoleParam)
         }

vmm/src/vm.rs

@@ -40,7 +40,7 @@ use pci::{
 use qcow::{self, ImageType, QcowFile};
 use std::ffi::CString;
 use std::fs::{File, OpenOptions};
-use std::io::{self, stdout};
+use std::io::{self, sink, stdout};
 use std::os::unix::fs::OpenOptionsExt;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::ptr::null_mut;
@@ -561,9 +561,9 @@ impl DeviceManager {
                     .map_err(DeviceManagerError::SerialOutputFileOpen)?,
             )),
             ConsoleOutputMode::Tty => Some(Box::new(stdout())),
-            ConsoleOutputMode::Off => None,
+            ConsoleOutputMode::Off | ConsoleOutputMode::Null => None,
         };
-        let serial = if serial_writer.is_some() {
+        let serial = if vm_info.vm_cfg.serial.mode != ConsoleOutputMode::Off {
             // Serial is tied to IRQ #4
             let serial_irq = 4;
             let interrupt: Box<devices::Interrupt> = if let Some(ioapic) = &ioapic {
@@ -578,9 +578,9 @@ impl DeviceManager {
                 Box::new(KernelIoapicIrq::new(serial_evt))
             };
 
-            Some(Arc::new(Mutex::new(devices::legacy::Serial::new_out(
+            Some(Arc::new(Mutex::new(devices::legacy::Serial::new(
                 interrupt,
-                serial_writer.unwrap(),
+                serial_writer,
             ))))
         } else {
             None
@@ -601,6 +601,7 @@ impl DeviceManager {
                     .map_err(DeviceManagerError::ConsoleOutputFileOpen)?,
             )),
             ConsoleOutputMode::Tty => Some(Box::new(stdout())),
+            ConsoleOutputMode::Null => Some(Box::new(sink())),
             ConsoleOutputMode::Off => None,
         };
         let console = if console_writer.is_some() {