vhost_user_fs: add the ability to set slave req fd

This adds the missing part of supporting virtiofs dax on the slave end,
that is, receiving a socket pair fd from the master end to set up a
communication channel for sending setupmapping & removemapping messages.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>

src/bin/vhost_user_fs.rs

@@ -17,6 +17,7 @@ use std::sync::{Arc, RwLock};
 use std::{convert, error, fmt, io, process};
 
 use vhost_rs::vhost_user::message::*;
+use vhost_rs::vhost_user::SlaveFsCacheReq;
 use vhost_user_backend::{VhostUserBackend, VhostUserDaemon, Vring};
 use vhost_user_fs::descriptor_utils::{Reader, Writer};
 use vhost_user_fs::filesystem::FileSystem;
@@ -72,6 +73,8 @@ struct VhostUserFsBackend<F: FileSystem + Send + Sync + 'static> {
     mem: Option<GuestMemoryMmap>,
     kill_evt: EventFd,
     server: Arc<Server<F>>,
+    // handle request from slave to master
+    vu_req: Option<SlaveFsCacheReq>,
 }
 
 impl<F: FileSystem + Send + Sync + 'static> Clone for VhostUserFsBackend<F> {
@@ -80,6 +83,7 @@ impl<F: FileSystem + Send + Sync + 'static> Clone for VhostUserFsBackend<F> {
             mem: self.mem.clone(),
             kill_evt: self.kill_evt.try_clone().unwrap(),
             server: self.server.clone(),
+            vu_req: self.vu_req.clone(),
         }
     }
 }
@@ -90,6 +94,7 @@ impl<F: FileSystem + Send + Sync + 'static> VhostUserFsBackend<F> {
             mem: None,
             kill_evt: EventFd::new(EFD_NONBLOCK).map_err(Error::CreateKillEventFd)?,
             server: Arc::new(Server::new(fs)),
+            vu_req: None,
         })
     }
 
@@ -105,7 +110,7 @@ impl<F: FileSystem + Send + Sync + 'static> VhostUserFsBackend<F> {
 
             let total = self
                 .server
-                .handle_message(reader, writer)
+                .handle_message(reader, writer, self.vu_req.as_mut())
                 .map_err(Error::ProcessQueue)?;
 
             used_desc_heads[used_count] = (head_index, total);
@@ -173,6 +178,10 @@ impl<F: FileSystem + Send + Sync + 'static> VhostUserBackend for VhostUserFsBack
     fn exit_event(&self) -> Option<(EventFd, Option<u16>)> {
         Some((self.kill_evt.try_clone().unwrap(), Some(KILL_EVENT)))
     }
+
+    fn set_slave_req_fd(&mut self, vu_req: SlaveFsCacheReq) {
+        self.vu_req = Some(vu_req);
+    }
 }
 
 fn main() {