implement p9fs read as direct host file to guest mem write

lib/propolis/src/hw/virtio/p9fs.rs

@@ -5,9 +5,9 @@
 use std::collections::HashMap;
 use std::convert::TryInto;
 use std::fs;
-use std::io::{Read, Seek};
 use std::mem::size_of;
 use std::num::NonZeroU16;
+use std::os::fd::AsRawFd;
 use std::os::unix::ffi::OsStrExt;
 use std::os::unix::fs::MetadataExt;
 use std::path::PathBuf;
@@ -332,7 +332,7 @@ impl HostFSHandler {
         mem: &MemCtx,
         msize: u32,
     ) {
-        let mut file = match fid.file {
+        let file = match fid.file {
             Some(ref f) => f,
             None => {
                 // the file is not open
@@ -366,15 +366,6 @@ impl HostFSHandler {
             return write_buf(buf, chain, mem);
         }
 
-        match file.seek(std::io::SeekFrom::Start(msg.offset)) {
-            Err(e) => {
-                let ecode = e.raw_os_error().unwrap_or(0);
-                warn!(self.log, "read: seek: {:?}: {:?}", &fid.pathbuf, e,);
-                return write_error(ecode as u32, chain, mem);
-            }
-            Ok(_) => {}
-        }
-
         let read_count = u32::min(msize, msg.count);
 
         let space_left = read_count as usize
@@ -387,22 +378,7 @@ impl HostFSHandler {
         let buflen =
             std::cmp::min(space_left, (metadata.len() - msg.offset) as usize);
 
-        let mut content: Vec<u8> = vec![0; buflen];
-
-        match file.read_exact(content.as_mut_slice()) {
-            Err(e) => {
-                let ecode = e.raw_os_error().unwrap_or(0);
-                warn!(self.log, "read: exact: {:?}: {:?}", &fid.pathbuf, e,);
-                return write_error(ecode as u32, chain, mem);
-            }
-            Ok(()) => {}
-        }
-
-        let response = Rread::new(content);
-        let mut out = ispf::to_bytes_le(&response).unwrap();
-        let buf = out.as_mut_slice();
-
-        write_buf(buf, chain, mem);
+        p9_write_file(&file.as_raw_fd(), chain, mem, buflen, msg.offset as i64);
     }
 
     fn do_statfs(&self, fid: &mut Fid, chain: &mut Chain, mem: &MemCtx) {
@@ -974,3 +950,54 @@ pub(crate) fn write_error(ecode: u32, chain: &mut Chain, mem: &MemCtx) {
     let buf = out.as_mut_slice();
     write_buf(buf, chain, mem);
 }
+
+pub(crate) fn p9_write_file(
+    file: &impl std::os::fd::AsRawFd,
+    chain: &mut Chain,
+    mem: &MemCtx,
+    count: usize,
+    offset: i64,
+) {
+    // Form the rread header. Unfortunately we can't do this with the Rread
+    // structure because the count is baked into the data field which is tied
+    // to the length of the vector and filling that vector is what we're
+    // explicitly trying to avoid here.
+    let sz = size_of::<u32>() + // size
+            size_of::<u8>()  + // typ
+            size_of::<u16>() + // tag
+            size_of::<u32>() +  // data.count
+            count; // data
+    let mut header = Vec::with_capacity(11);
+    header.extend_from_slice(&(sz as u32).to_le_bytes());
+    header.push(p9ds::proto::MessageType::Rread as u8);
+    header.extend_from_slice(&0u16.to_le_bytes());
+    header.extend_from_slice(&(count as u32).to_le_bytes());
+
+    // Send the header to the guest from the buffer constructed above. Then
+    // send the actual file data
+    let mut header_done = false;
+    let mut done = 0;
+    let _total = chain.for_remaining_type(false, |addr, len| {
+        let mut remain = len;
+        let mut copied = 0;
+        if !header_done {
+            mem.write_from(addr, &header, header.len()).unwrap();
+            header_done = true;
+            remain -= header.len();
+            copied += header.len();
+        }
+        let addr = GuestAddr(addr.0 + copied as u64);
+        let sub_mapping =
+            mem.direct_writable_region(&GuestRegion(addr, remain)).unwrap();
+
+        let len = usize::min(remain, count);
+        let off = offset + done as i64;
+        let mapped = sub_mapping.pread(file, len, off).unwrap();
+        copied += mapped;
+        done += mapped;
+
+        let need_more = done < count;
+
+        (copied, need_more)
+    });
+}

lib/propolis/src/hw/virtio/queue.rs

@@ -752,7 +752,6 @@ pub mod migrate {
     }
 }
 
-#[cfg(feature = "falcon")]
 pub(crate) fn write_buf(buf: &[u8], chain: &mut Chain, mem: &MemCtx) {
     // more copy pasta from Chain::write b/c like Chain:read a
     // statically sized type is expected.