qemu/kvm hangs reading from serial console

[Buck]

This is for a qemu-kvm running on RHEL 5, so it's pretty old,
but i think the problem still exists in 1.2

We have conman running on our hosts, connecting to the
kvm/qemu's using
    virsh console
which just opens up the console /dev/pts/slave that qemu
opens up when run with options
    -nographic
    -serial mon:pty

Sometimes virsh console exits and then qemu locks up[*].
My guess is that something like this happens:

virsh console exits
qemu does a select() on /dev/ptmx (and other FDs)
select() returns the FD of /dev/ptmx in the read-fdset
qemu does a read()
read() returns -1 (EIO)
qemu does other stuff for a while
select() ... /dev/ptmx
read() .. EIO
other stuff
select() ... read() ... select() ... read() ... select()
conman starts a new virsh console that connects
qemu does a read()
read() blocks b/c there is now a writer on the tty slave

So i don't see any way around this, given the sorta rudi-
mentary semantics of TTY IO on Linux (not that i know of
any platform that does it better ... ?), except ...

maybe qemu should
    fcntl(master_fd, F_SETFL, flags | O_NONBLOCK)
in qemu-char.c:qemu_char_open_pty()
and be prepared to handle E_WOULDBLOCK|E_AGAIN in
qemu-char.c:fd_chr_read() ... ?

--buck

[*] i think, b/c in the old version we are running, sometimes
    the guest spits out the
        ^]
    character to its console, and virsh console reads it and
    doesn't check to see if its from stdin or the pty and exits,
    which, i think, can be fixed like this:

--- libvirt-0.8.2/tools/console.c.ctrl_close_bracket_handling_fix 2012-09-06 10:30:43.606997191 -0400
+++ libvirt-0.8.2/tools/console.c 2012-09-06 10:34:52.154000464 -0400
@@ -155,6 +155,7 @@ int vshRunConsole(const char *tty) {

                 /* Quit if end of file, or we got the Ctrl-] key */
                 if (!got ||
+ fds[i].fd == STDIN_FILENO &&
                     (got == 1 &&
                      buf[0] == CTRL_CLOSE_BRACKET))
                     goto done;

[Thomas Huth (th-huth)]

Can you still reproduce this problem with the latest version of QEMU? There have been quite a bunch of fixes in this area in the latest versions...

[Buck (buckh)]

i don't use kvm/qemu any more and so don't have a means
to determine if this is still an issue or not so please
presume fixed, i guess

[Thomas Huth (th-huth)]

OK, thanks for your answer!