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[Qemu-devel] [Bug 1047470] Re: qemu/kvm hangs reading from serial consol
From: |
Buck |
Subject: |
[Qemu-devel] [Bug 1047470] Re: qemu/kvm hangs reading from serial console |
Date: |
Fri, 07 Sep 2012 18:07:28 -0000 |
** 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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