misc: Make Bus::write() return an Option<Arc<Barrier>>

This can be uses to indicate to the caller that it should wait on the
barrier before returning as there is some asynchronous activity
triggered by the write which requires the KVM exit to block until it's
completed.

This is useful for having vCPU thread wait for the VMM thread to proceed
to activate the virtio devices.

See #1863

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

devices/src/legacy/cmos.rs

@@ -5,6 +5,7 @@
 use libc::{clock_gettime, gmtime_r, time_t, timespec, tm, CLOCK_REALTIME};
 use std::cmp::min;
 use std::mem;
+use std::sync::{Arc, Barrier};
 use vm_device::BusDevice;
 
 const INDEX_MASK: u8 = 0x7f;
@@ -44,16 +45,17 @@ impl Cmos {
 }
 
 impl BusDevice for Cmos {
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() != 1 {
-            return;
+            return None;
         }
 
         match offset {
             INDEX_OFFSET => self.index = data[0] & INDEX_MASK,
             DATA_OFFSET => self.data[self.index as usize] = data[0],
             o => panic!("bad write offset on CMOS device: {}", o),
-        }
+        };
+        None
     }
 
     fn read(&mut self, _base: u64, offset: u64, data: &mut [u8]) {

devices/src/legacy/fwdebug.rs

@@ -7,6 +7,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 //
 
+use std::sync::{Arc, Barrier};
 use vm_device::BusDevice;
 
 /// Provides firmware debug output via I/O port controls
@@ -30,11 +31,13 @@ impl BusDevice for FwDebugDevice {
         }
     }
 
-    fn write(&mut self, _base: u64, _offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, _offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() == 1 {
             print!("{}", data[0] as char);
         } else {
             error!("Invalid write size on debug port: {}", data.len())
         }
+
+        None
     }
 }

devices/src/legacy/i8042.rs

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE-BSD-3-Clause file.
 
-use vmm_sys_util::eventfd::EventFd;
-
+use std::sync::{Arc, Barrier};
 use vm_device::BusDevice;
+use vmm_sys_util::eventfd::EventFd;
 
 /// A i8042 PS/2 controller that emulates just enough to shutdown the machine.
 pub struct I8042Device {
@@ -32,12 +32,14 @@ impl BusDevice for I8042Device {
         }
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() == 1 && data[0] == 0xfe && offset == 3 {
             debug!("i8042 reset signalled");
             if let Err(e) = self.reset_evt.write(1) {
                 error!("Error triggering i8042 reset event: {}", e);
             }
         }
+
+        None
     }
 }

devices/src/legacy/rtc_pl031.rs

@@ -9,7 +9,7 @@
 //! a real-time clock input.
 //!
 use std::fmt;
-use std::sync::Arc;
+use std::sync::{Arc, Barrier};
 use std::time::Instant;
 use std::{io, result};
 use vm_device::interrupt::InterruptSourceGroup;
@@ -371,7 +371,7 @@ impl BusDevice for RTC {
         }
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() <= 4 {
             let v = read_le_u32(&data[..]);
             if let Err(e) = self.handle_write(offset, v) {
@@ -384,6 +384,8 @@ impl BusDevice for RTC {
                 data.len()
             );
         }
+
+        None
     }
 }
 

devices/src/legacy/serial.rs

@@ -7,7 +7,7 @@
 
 use anyhow::anyhow;
 use std::collections::VecDeque;
-use std::sync::Arc;
+use std::sync::{Arc, Barrier};
 use std::{io, result};
 use vm_device::interrupt::InterruptSourceGroup;
 use vm_device::BusDevice;
@@ -275,12 +275,14 @@ impl BusDevice for Serial {
         };
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if data.len() != 1 {
-            return;
+            return None;
         }
 
-        if let Err(_e) = self.handle_write(offset as u8, data[0]) {}
+        self.handle_write(offset as u8, data[0]).ok();
+
+        None
     }
 }