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>

pci/src/bus.rs

@@ -10,7 +10,7 @@ use byteorder::{ByteOrder, LittleEndian};
 use std::any::Any;
 use std::collections::HashMap;
 use std::ops::DerefMut;
-use std::sync::{Arc, Mutex};
+use std::sync::{Arc, Barrier, Mutex};
 use vm_device::{Bus, BusDevice};
 use vm_memory::{Address, GuestAddress, GuestUsize};
 
@@ -312,13 +312,15 @@ impl BusDevice for PciConfigIo {
         }
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         // `offset` is relative to 0xcf8
         match offset {
             o @ 0..=3 => self.set_config_address(o, data),
             o @ 4..=7 => self.config_space_write(o - 4, data),
             _ => (),
         };
+
+        None
     }
 }
 
@@ -407,11 +409,13 @@ impl BusDevice for PciConfigMmio {
         }
     }
 
-    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) {
+    fn write(&mut self, _base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
         if offset > u64::from(u32::max_value()) {
-            return;
+            return None;
         }
-        self.config_space_write(offset as u32, offset % 4, data)
+        self.config_space_write(offset as u32, offset % 4, data);
+
+        None
     }
 }
 

pci/src/vfio.rs

@@ -15,7 +15,7 @@ use std::any::Any;
 use std::ops::Deref;
 use std::os::unix::io::AsRawFd;
 use std::ptr::null_mut;
-use std::sync::Arc;
+use std::sync::{Arc, Barrier};
 use std::{fmt, io, result};
 use vfio_bindings::bindings::vfio::*;
 use vfio_ioctls::{VfioDevice, VfioError};
@@ -652,8 +652,10 @@ impl BusDevice for VfioPciDevice {
         self.read_bar(base, offset, data)
     }
 
-    fn write(&mut self, base: u64, offset: u64, data: &[u8]) {
-        self.write_bar(base, offset, data)
+    fn write(&mut self, base: u64, offset: u64, data: &[u8]) -> Option<Arc<Barrier>> {
+        self.write_bar(base, offset, data);
+
+        None
     }
 }