Re: [PATCH] console: make QMP screendump use coroutine

[Markus Armbruster <armbru@redhat.com>]

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 ;)