vhost_user_fs: add fs cache request operations

This introduces setupmapping and removemapping methods to server.rs,
passthrough.rs and filesystem.rs in order to support virtiofs dax mode
inside guest.

Since we don't really want the server.rs to know that it is dealing with
vhost-user specifically, this is making it more generic by adding a new
trait which has three functions map()/unmap()/sync() corresponding to
fs_slave_{map, unmap, sync}, server.rs will take anything that implements
the trait.

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

vhost_user_fs/src/fs_cache_req_handler.rs

@@ -0,0 +1,61 @@
+use crate::fuse;
+use std::io;
+use std::os::unix::io::RawFd;
+use vhost_rs::vhost_user::message::{
+    VhostUserFSSlaveMsg, VhostUserFSSlaveMsgFlags, VHOST_USER_FS_SLAVE_ENTRIES,
+};
+use vhost_rs::vhost_user::{SlaveFsCacheReq, VhostUserMasterReqHandler};
+
+/// Trait for virtio-fs cache requests operations.  This is mainly used to hide
+/// vhost-user details from virtio-fs's fuse part.
+pub trait FsCacheReqHandler: Send + Sync + 'static {
+    /// Setup a dedicated mapping so that guest can access file data in DAX style.
+    fn map(
+        &mut self,
+        foffset: u64,
+        moffset: u64,
+        len: u64,
+        flags: u64,
+        fd: RawFd,
+    ) -> io::Result<()>;
+
+    /// Remove those mappings that provide the access to file data.
+    fn unmap(&mut self, requests: Vec<fuse::RemovemappingOne>) -> io::Result<()>;
+}
+
+impl FsCacheReqHandler for SlaveFsCacheReq {
+    fn map(
+        &mut self,
+        foffset: u64,
+        moffset: u64,
+        len: u64,
+        flags: u64,
+        fd: RawFd,
+    ) -> io::Result<()> {
+        let mut msg: VhostUserFSSlaveMsg = Default::default();
+        msg.fd_offset[0] = foffset;
+        msg.cache_offset[0] = moffset;
+        msg.len[0] = len;
+        msg.flags[0] = if (flags & fuse::SetupmappingFlags::WRITE.bits()) != 0 {
+            VhostUserFSSlaveMsgFlags::MAP_W | VhostUserFSSlaveMsgFlags::MAP_R
+        } else {
+            VhostUserFSSlaveMsgFlags::MAP_R
+        };
+
+        self.fs_slave_map(&msg, fd)
+    }
+
+    fn unmap(&mut self, requests: Vec<fuse::RemovemappingOne>) -> io::Result<()> {
+        for chunk in requests.chunks(VHOST_USER_FS_SLAVE_ENTRIES) {
+            let mut msg: VhostUserFSSlaveMsg = Default::default();
+
+            for (ind, req) in chunk.iter().enumerate() {
+                msg.len[ind] = req.len;
+                msg.cache_offset[ind] = req.moffset;
+            }
+
+            self.fs_slave_unmap(&msg)?;
+        }
+        Ok(())
+    }
+}