virtio-console: Redirect init/kernel output to rust log

When we redirect the stdout of the guest it opens the question,
where should init/kernel messages (things written to the console) go.
It seems to make sense to write it as errors to the log,
because for now the only messages that go there are errors.

Author: mtjhrc

init/init.c

@@ -918,6 +918,7 @@ int main(int argc, char **argv)
     }
 
 	if (execvp(exec_argv[0], exec_argv) < 0) {
+		//TODO: maybe print this msg to the console not redirected stdout?
 		printf("Couldn't execute '%s' inside the vm: %s\n", exec_argv[0], strerror(errno));
 		exit(-3);
 	}

src/devices/src/virtio/console/port_io.rs

@@ -2,6 +2,7 @@ use std::io::{self, ErrorKind};
 use std::os::fd::{AsRawFd, FromRawFd, OwnedFd, RawFd};
 
 use libc::{fcntl, F_GETFL, F_SETFL, O_NONBLOCK, STDERR_FILENO, STDIN_FILENO, STDOUT_FILENO};
+use log::Level;
 use nix::errno::Errno;
 use nix::poll::{poll, PollFd, PollFlags};
 use nix::unistd::dup;
@@ -40,6 +41,10 @@ pub fn output_to_raw_fd_dup(fd: RawFd) -> Result<Box<dyn PortOutput + Send>, nix
     Ok(Box::new(PortOutputFd(fd)))
 }
 
+pub fn output_to_log_as_err() -> Box<dyn PortOutput + Send> {
+    Box::new(PortOutputLog::new())
+}
+
 struct PortInputFd(OwnedFd);
 
 impl AsRawFd for PortInputFd {
@@ -129,3 +134,48 @@ fn make_non_blocking(as_rw_fd: &impl AsRawFd) -> Result<(), nix::Error> {
     }
     Ok(())
 }
+
+// Utility to relay log from the VM (the kernel boot log and messages from init)
+// to the rust log
+#[derive(Default)]
+pub struct PortOutputLog {
+    buf: Vec<u8>,
+}
+
+impl PortOutputLog {
+    const FORCE_FLUSH_TRESHOLD: usize = 512;
+    const LOG_TARGET: &'static str = "init_or_kernel";
+
+    fn new() -> Self {
+        Self::default()
+    }
+
+    fn force_flush(&mut self) {
+        log::log!(target: PortOutputLog::LOG_TARGET, Level::Error, "[missing newline]{}", String::from_utf8_lossy(&self.buf));
+        self.buf.clear();
+    }
+}
+
+impl PortOutput for PortOutputLog {
+    fn write_volatile(&mut self, buf: &VolatileSlice) -> Result<usize, io::Error> {
+        self.buf
+            .write_volatile(buf)
+            .map_err(|e| io::Error::new(ErrorKind::Other, e))?;
+
+        let mut start = 0;
+        for (i, ch) in self.buf.iter().cloned().enumerate() {
+            if ch == b'\n' {
+                log::log!(target: PortOutputLog::LOG_TARGET, Level::Error, "{}", String::from_utf8_lossy(&self.buf[start..i]));
+                start = i + 1;
+            }
+        }
+        self.buf.drain(0..start);
+        // Make sure to not grow the internal buffer forever!
+        if self.buf.len() > PortOutputLog::FORCE_FLUSH_TRESHOLD {
+            self.force_flush()
+        }
+        Ok(buf.len())
+    }
+
+    fn wait_until_writable(&self) {}
+}

src/vmm/src/builder.rs

@@ -1061,7 +1061,7 @@ fn attach_console_devices(
         output: if stdout_is_terminal {
             Some(port_io::stdout().unwrap())
         } else {
-            None
+            Some(port_io::output_to_log_as_err())
         },
     }];