vhost-user-backend: Add vhost-user bitmap trait bounds

Any bitmap used in the vhost-user backend must implement the
BitmapReplace trait, which provides the functionality to replace the
internal bitmap in runtime.

This internal bitmap is required because in the vm-memory crate the
bitmap is expected to exist at the time of creating the memory regions,
and in the case of vhost-user the bitmap is added at runtime, also it
could be replaced at a later time. In addition, the vhost user protocol
does not specify whether the previous bitmap is still active after
replying to the VHOST_USER_SET_LOG_BASE message, so we must be sure
that the in-flight requests are using the new bitmap after the message
reply.

Signed-off-by: German Maglione <[email protected]>

Author: germag

vhost-user-backend/src/handler.rs

@@ -12,6 +12,7 @@ use std::os::unix::io::AsRawFd;
 use std::sync::Arc;
 use std::thread;
 
+use crate::bitmap::BitmapReplace;
 #[cfg(feature = "postcopy")]
 use userfaultfd::{Uffd, UffdBuilder};
 use vhost::vhost_user::message::{
@@ -230,7 +231,7 @@ where
 
 impl<T: VhostUserBackend> VhostUserBackendReqHandlerMut for VhostUserHandler<T>
 where
-    T::Bitmap: NewBitmap + Clone,
+    T::Bitmap: BitmapReplace + NewBitmap + Clone,
 {
     fn set_owner(&mut self) -> VhostUserResult<()> {
         if self.owned {

vhost-user-backend/src/lib.rs

@@ -29,6 +29,7 @@ mod handler;
 pub use self::handler::VhostUserHandlerError;
 
 pub mod bitmap;
+use crate::bitmap::BitmapReplace;
 
 mod vring;
 pub use self::vring::{
@@ -97,7 +98,7 @@ pub struct VhostUserDaemon<T: VhostUserBackend> {
 impl<T> VhostUserDaemon<T>
 where
     T: VhostUserBackend + Clone + 'static,
-    T::Bitmap: NewBitmap + Clone + Send + Sync,
+    T::Bitmap: BitmapReplace + NewBitmap + Clone + Send + Sync,
     T::Vring: Clone + Send + Sync,
 {
     /// Create the daemon instance, providing the backend implementation of `VhostUserBackend`.