[Qemu-devel] Re: [PATCH 0/5] chardev, virtio-console: flow control, error handling

[Gerd Hoffmann]

  Hi,

> In case of EAGAIN, the unix_write() function just kept spinning while
> attempting to write to the chardev till it succeeded. This resulted in
> a stuck VM in case a chardev had opened connection but wasn't reading
> anything from qemu.

It spins only for non-blocking file handles.  In blocking mode the write 
syscall just goes sleep until it can write out data (or gets a signal).

> There are two fixes for that case:
> - Poll for POLLOUT instead of directly attempting write(), which helps
>    relax the CPU (and we become greener).

No need to do that, see above.  Having a poll() call in unix_write() is 
wrong IMHO.

> - If the file that we're writing to is nonblocking, return -EAGAIN to
>    the caller of qemu_chr_write() so that appropriate actions can be
>    taken higher up in the stack.

Good.

cheers,
  Gerd