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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 2/2] main: switch qemu_set_fd_handler to g_io_add_watch
Date: Wed, 07 Sep 2011 16:40:39 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:6.0) Gecko/20110816 Thunderbird/6.0

On 09/07/2011 02:42 PM, Anthony Liguori wrote:
On 09/07/2011 02:03 AM, Paolo Bonzini wrote:
On 09/06/2011 05:59 PM, Anthony Liguori wrote:
So it should be possible to add a new Source type that just selects on a
file descriptor and avoid GIOChannels?

I think you still have the problem that glib on Windows waits for
HANDLEs, not file descriptors. Also, I'm not sure it's worth it though
as long as slirp still does its own fill/poll.

So how do we fix this long term?

Long term, we use GIOChannels for everything, assuming that's possible at all. More realistically, we could rewrite socket handling on Windows so that we can use g_poll instead of select (don't wait for me doing that).

Another possibility, the ugliest but also the most realistic, is to separate the Windows and POSIX implementations of the main loop more sharply. This way glib's main loop can be integrated (differently) into both implementations.

In the meanwhile: just do not rely on glib sources on Windows. There isn't any large benefit in this patch, and it actually complicates the straightforward code in iohandler. Just revert it and #ifdef the glib integration in patch 1/2. Since I don't see a 100%-glib main loop anytime soon, we are unlikely to lose much. If anybody introduces a feature that requires Avahi or GTK+, it won't be supported on Windows.

We seem to get away with using fds
today and not HANDLEs, do fds on Windows not work the same with poll as
they do with select?

Here is a summary table:

select                    socket HANDLEs only
poll                      does not exist
WaitForMultipleObjects     all other HANDLEs
g_poll                    all other HANDLEs

We only use select for Windows socket handles today. Everything else is handled separately (with WaitForMultipleObjects) by osdep-win32.c/oslib-win32.c.


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