Re: [Qemu-devel] [PATCH 2/2] main: switch qemu_set_fd_handler to g_io_add_watch

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/25/2011 11:24 AM, Fabien Chouteau wrote:
>> For POSIX, it would be just a call to
>> glib_select_fill+select+glib_select_poll.  (Everything around
>> these three would stay in the caller, and the fd_sets would be
>> passed to os_host_main_loop_wait).
>
> Are you sure we have to use select()?

slirp is fd_set---thus select()---based.  iohandler too, though it would 
likely be simpler to switch it to poll().

> I would expect Glib to help us
> avoid this kind of os-dependent syscalls.

Long term, yes.  However, even with the iothread and other recent 
refactorings, the QEMU event loop is still in control of everything 
including glib sources.  This is not a problem; the glib event loop is 
designed to be integrated into other event loops.

> Again, Glib should help us skip all these complicated os-dependent
> stuff.

To do this, you need to reimplement the various components of the QEMU 
main loop as GSources.  I did it for eventfd (including bottom halves), 
timerfd and signalfd, but really it was only to learn GSources rather 
than as something planned for inclusion.

What's missing is iohandlers and qemu_aio_wait/flush are needed too. 
Either this, or you would need to touch all uses of iohandlers: 
character devices, the non-raw block protocols (nbd, curl, iscsi, etc.), 
slirp, and migration.

If you do not want to do this all at once, the first step is to fix the 
glib main loop for Windows and move things over slowly.

Paolo