[Qemu-devel] [Bug 1047470] Re: qemu/kvm hangs reading from serial console

[Buck]

** Description changed:

  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
+     virsh console
  which just opens up the console /dev/pts/slave that qemu
  opens up when run with options
-     -nographic
-     -serial mon:pty
+     -nographic
+     -serial mon:pty
  
- Sometimes virsh console exits and then qemu locks up.
+ 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) 
+     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 
+ 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:
+     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 ||
+                  /* 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;
+                      (got == 1 &&
+                       buf[0] == CTRL_CLOSE_BRACKET))
+                      goto done;

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https://bugs.launchpad.net/bugs/1047470

Title:
  qemu/kvm hangs reading from serial console

Status in QEMU:
  New

Bug description:
  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;

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