qemu-ga: add initial win32 support

This adds a win32 channel implementation that makes qemu-ga functional
on Windows using virtio-serial (unix-listen/isa-serial not currently
implemented). Unlike with the posix implementation, we do not use
GIOChannel for the following reasons:

 - glib calls stat() on an fd to check whether S_IFCHR is set, which is
   the case for virtio-serial on win32. Because of that, a one-time
   check to determine whether the channel is readable is done by making
   a call to PeekConsoleInput(), which reports the underlying handle is
   not a valid console handle, and thus we can never read from the
   channel.

 - if one goes as far as to "trick" glib into thinking it is a normal
   file descripter, the buffering is done in such a way that data
   written to the output stream will subsequently result in that same
   data being read back as if it were input, causing an error loop.
   furthermore, a forced flush of the channel only moves the data into a
   secondary buffer managed by glib, so there's no way to prevent output
   from getting read back as input.

The implementation here ties into the glib main loop by implementing a
custom GSource that continually submits asynchronous/overlapped I/O to
fill an GAChannel-managed read buffer, and tells glib to poll the
corresponding event handle for a completion whenever there is no
data/RPC in the read buffer to notify the main application about.

1 files changed, 4 insertions, 0 deletions

diff --git a/qemu-ga.c b/qemu-ga.c
index 93ebc3e47..8e517b551 100644
--- a/qemu-ga.c
+++ b/qemu-ga.c
@@ -30,7 +30,11 @@
 #include "qapi/qmp-core.h"
 #include "qga/channel.h"
 
+#ifndef _WIN32
 #define QGA_VIRTIO_PATH_DEFAULT "/dev/virtio-ports/org.qemu.guest_agent.0"
+#else
+#define QGA_VIRTIO_PATH_DEFAULT "\\\\.\\Global\\org.qemu.guest_agent.0"
+#endif
 #define QGA_PIDFILE_DEFAULT "/var/run/qemu-ga.pid"
 
 struct GAState {