I tried to observe the main loop keeps running while the screendump does
its work.
The main loop appears to lack trace points. Alright, if there's no
hammer handy, I'll use a rock:
diff --git a/softmmu/vl.c b/softmmu/vl.c
index 5549f4b619..b6561a65d7 100644
--- a/softmmu/vl.c
+++ b/softmmu/vl.c
@@ -1661,6 +1661,7 @@ void qemu_main_loop(void)
#ifdef CONFIG_PROFILER
ti = profile_getclock();
#endif
+ printf("*** main loop\n");
main_loop_wait(false);
#ifdef CONFIG_PROFILER
dev_time += profile_getclock() - ti;
First experiment: does the main loop continue to run when writing out
the screendump blocks / would block?
Observe qmp_screendump() opens the file without O_EXCL. Great, that
lets me block output by making it open a FIFO.
Terminal#1:
$ mkfifo s
Terminal#2:
$ upstream-qemu -S -display none -chardev
socket,id=qmp,path=test-qmp,server=on,wait=off -mon mode=control,chardev=qmp
*** main loop
*** main loop
*** main loop
Keeps printing at a steady pace.
Terminal#3:
$ socat "READLINE,history=$HOME/.qmp_history,prompt=QMP>"
UNIX-CONNECT:$HOME/work/images/test-qmp
{"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 4},
"package": "v4.2.0-2069-g5e5ae6b644-dirty"}, "capabilities": ["oob"]}}
QMP>{"execute": "qmp_capabilities"}
{"return": {}}
QMP>{"execute": "screendump", "arguments": {"filename": "s"}}
The printing in terminal#2 stops. This is expected; qemu_open() calls
open(), which blocks, because the FIFO has no reader.
Terminal#1:
$ exec 4<s
Now the FIFO has a reader. Terminal#2 remains quiet.
We now hang in ppm_save(). Abridged stack backtrace:
#0 0x00007ffff519d0f5 in writev () at /lib64/libc.so.6
#1 0x0000555555e15f61 in qio_channel_file_writev
(ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, fds=0x0, nfds=0,
errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel-file.c:123
#2 0x0000555555e133d3 in qio_channel_writev_full
(ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, fds=0x0, nfds=0,
errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel.c:86
#3 0x0000555555e137a2 in qio_channel_writev
(ioc=0x5555567bf5f0, iov=0x555556a441b0, niov=1, errp=0x7fffe9d81d10)
at /work/armbru/qemu/io/channel.c:207
#4 0x0000555555e13696 in qio_channel_writev_all
(ioc=0x5555567bf5f0, iov=0x7fffe9d81bd0, niov=1, errp=0x7fffe9d81d10)
at /work/armbru/qemu/io/channel.c:171
#5 0x0000555555e139b1 in qio_channel_write_all
(ioc=0x5555567bf5f0, buf=0x555556b05200 "", buflen=1920,
errp=0x7fffe9d81d10) at /work/armbru/qemu/io/channel.c:257
#6 0x0000555555cd74ff in ppm_save
(fd=22, image=0x5555568ffdd0, errp=0x7fffe9d81d10)
at /work/armbru/qemu/ui/console.c:336
#7 0x0000555555cd77e6 in qmp_screendump
(filename=0x555556ea0900 "s", has_device=false, device=0x0,
has_head=false, head=0, errp=0x7fffe9d81d10) at
/work/armbru/qemu/ui/console.c:401
A brief inspection of qio_channel_file_writev() and
qio_channel_writev_all() suggests this might work if you make the output
file descriptor non-blocking.
$ head -c 1 <&4 | hexdump -C
00000000 50 |P|
00000001
Still quiet.
$ cat <&4 >/dev/null
The printing resumes.
$ exec 4<&-
Second experiment: does the main loop continue to run while we wait for
graphic_hw_update_done()?
Left as an exercise for the patch submitter ;)
