• Marc-André Lureau's avatar
    win32: work around main-loop busy loop on socket/fd event · 873df2ce
    Marc-André Lureau authored
    Commit 05e514b1 introduced an AIO
    context optimization to avoid calling event_notifier_test_and_clear() on
    ctx->notifier. On Windows, the same notifier is being used to wakeup the
    wait on socket events (see commit
    d3385eb4).
    
    The ctx->notifier event is added to the gpoll sources in
    aio_set_event_notifier(), aio_ctx_check() should clear the event
    regardless of ctx->notified, since Windows sets the event by itself,
    bypassing the aio->notified. This fixes qemu not clearing the event
    resulting in a busy loop.
    
    Paolo suggested to me on irc to call event_notifier_test_and_clear()
    after select() >0 from aio-win32.c's aio_prepare. Unfortunately, not all
    fds associated with ctx->notifiers are in AIO fd handlers set.
    (qemu_set_nonblock() in util/oslib-win32.c calls qemu_fd_register()).
    
    This is essentially a v2 of a patch that was sent earlier:
    https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg00420.html
    
    that resurfaced when James investigated Spice performance issues on Windows:
    https://gitlab.freedesktop.org/spice/spice/issues/36
    
    In order to test that patch, I simply tried running test-char on
    win32, and it hangs. Applying that patch solves it. QIO idle sources
    are not dispatched. I haven't investigated much further, I suspect
    source priorities and busy looping still come into play.
    
    This version keeps the "notified" field, so event_notifier_poll()
    should still work as expected.
    
    Cc: James Le Cuirot <chewi@gentoo.org>
    Signed-off-by: default avatarMarc-André Lureau <marcandre.lureau@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    873df2ce
async.c 13.3 KB