QEMUFileBuffered stops writing when the underlying QEMUFile is not
ready,
and tells its producer so. However, when the underlying QEMUFile
becomes
ready, it neglects to pass that information along, resulting in stoppage
of all data until the next tick (a tenths of a second).
Usually this doesn't matter, because most QEMUFiles used with
QEMUFileBuffered
are almost always ready, but in the case of exec: migration this is
not true,
due to the small pipe buffers used to connect to the target process.
The
result is very slow migration.
Fix by detecting the readiness notification and propagating it. The
detection
is a little ugly since QEMUFile overloads put_buffer() to send it,
but that's
the suject for a different patch.
Signed-off-by: Avi Kivity<address@hidden>\