When testing with the new "-M none" arg, I've noticed that ~70% of the time libvirt starts QEMU will result in a SEGV from QEMU with the following stack trace:
(gdb) bt #0 0x0000000000000000 in ?? () #1 0x000055555567a37f in json_lexer_feed_char (lexer=0x55555658fb20, ch=123 '{', flush=false) at json-lexer.c:324 #2 0x000055555567a4aa in json_lexer_feed (lexer=0x55555658fb20, buffer=0x7fffffffe7b7 "{", size=1) at json-lexer.c:356 #3 0x000055555567c708 in json_message_parser_feed (parser=0x55555658fb18, buffer=0x7fffffffe7b7 "{", size=1) at json-streamer.c:110 #4 0x0000555555882861 in monitor_control_read (opaque=0x55555658f6a0, buf=0x7fffffffe7b7 "{", size=1) at /home/berrange/src/virt/qemu/monitor.c:4768 #5 0x000055555579b051 in qemu_chr_be_write (s=0x55555658dc10, buf=0x7fffffffe7b7 "{", len=1) at qemu-char.c:164 #6 0x000055555579c9c8 in stdio_read (opaque=0x55555658dc10) at qemu-char.c:720 #7 0x000055555567941f in qemu_iohandler_poll (readfds=0x5555560f17c0, writefds=0x5555560f1840, xfds=0x5555560f18c0, ret=2) at iohandler.c:122 #8 0x000055555577166a in main_loop_wait (nonblocking=0) at main-loop.c:497 #9 0x000055555576956b in main_loop () at /home/berrange/src/virt/qemu/vl.c:1643 #10 0x0000555555770239 in main (argc=10, argv=0x7fffffffeca8, envp=0x7fffffffed00) at /home/berrange/src/virt/qemu/vl.c:3755 Stack frame #1 there is doing this: lexer->emit(lexer, lexer->token, JSON_ERROR, lexer->x, lexer->y); GDB confirms that the 'emit' field has not yet been initialized. In the case of QMP, this is initialized by the following sequence: - main - chardev_init_func - qemu_chr_generic_open ...async from event loop... - main_loop - qemu_chr_generic_open_bh - monitor_control_event - json_message_parser_init - json_lexer_init The problem arises if you try to feed data to QEMU before the bottom half has run. There is a race where qemu_chr_be_write can be called to process input, before the qemu_chr_generic_open_bh has been invoked. This can actually be quite easily demonstrated (at least on my system): # echo "{" | qemu-system-x86_64 -nodefaults -nographic -M none -qmp stdio Segmentation fault If you remove the '-M none' call, you won't hit this race condition 99% of the time, but I have occassionally been able to see it. It isn't clear to me what to change to solve this race condition. Probably though, the I/O handlers for a char device should be registered until the open bottom half has completed. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|