Re: [Qemu-devel] [PATCH v3 2/4] char: Add ability to provide a callback when write won't return -EAGAIN

[Gerd Hoffmann <kraxel@redhat.com>]

>   static void tcp_chr_connect(void *opaque)
>   {

> +    chr->write_blocked = false;
>       s->connected = 1;
>       qemu_set_fd_handler2(s->fd, tcp_chr_read_poll,
> -                         tcp_chr_read, NULL, chr);
> +                         tcp_chr_read, tcp_chr_write_unblocked, chr);

This is wrong.  You want register tcp_chr_write_unblocked only for the 
chr->write_blocked == true (i.e. output buffers are full) case. 
Otherwise qemu will burn cpu calling tcp_chr_write_unblocked.

Yes, you'll have to call qemu_set_fd_handler2 each time write_blocked 
changes state.

Also implementing the whole logic at the individual chardev drivers 
level feels somewhat wrong as it will identical for most (all?) unix 
chardev drivers.

cheers,
   Gerd