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
