Re: [Qemu-devel] [PATCH v6 00/25] Fixing record/replay and adding reverse debugging

[Artem Pisarenko <artem.k.pisarenko@gmail.com>]

I've posted bug report with extended tests (incl. case without sleep=off).
You may find guest image (kernel) in bug description.
https://bugs.launchpad.net/qemu/+bug/1795369

The most annoying thing is that some issues are almost not reproducible.
There are definitely race conditions somewhere in qemu code. Running
'stress-ng' utility with CPU and I/O stressors in parallel with qemu
execution greatly minimizes amount of attempts when I'm trying to trigger
some of issues I encounter.

I'll try 'info monitor' command tomorrow, but no guarantees that I'll be
able to reproduce issue again.

Speaking about '-nographic' and SDL... I've noted that UI greatly minimizes
possibility of hanging (but not avoids it completely) when using icount in
general, so this effect isn't rr-specific. I've already reported this bug
too.


пн, 1 окт. 2018 г., 20:14 dovgaluk <dovgaluk@ispras.ru>:

> Artem Pisarenko писал 2018-09-30 14:01:
> > Feature still broken :(
>
> Thanks for testing.
>
> >
> > Brief description of my tests.
> >
> > Guest image is Linux, which just powers off after kernel boots
> > (instead of proceeding to user-space /init or /sbin/init).
> > Base cmdline:
> > qemu-system-x86_64 -nodefaults -machine pc,accel=tcg -m 2048 -cpu
> > qemu64 -rtc clock=vm,base=2000-01-01T00:00:00 -kernel bzImage -initrd
> > rootfs -append 'nokaslr console=ttyS0 rdinit=/init_poweroff'
> > -nographic -serial SERIAL_VALUE -icount
> > 1,sleep=off,rr=RR_VALUE,rrfile=icount_rr_capture.bin
>
> I've never tried it with sleep=off. Can you remove it and try again?
>
> We also seen a problem with '-nographic'. When we remove this option and
> QEMU runs with SDL
> window, everything is ok. There is some problem with main loop which may
> sleep when there
> is no GUI to update, or something like that. We couldn't fix it yet.
>
> >
> > Test 1. When SERIAL_VALUE=none
> > Running with RR_VALUE=record completes successfully.
> > Running with RR_VALUE=replay doesn't completes. qemu process just
> > eating ~100% cpu and memory usage doesn't grow after some moment. I
> > don't see what happens because of problem no.2 (see below).
>
> Try 'info replay' monitor command. Does instruction counter increases?
>
> >
> > Test 2. When SERIAL_VALUE=stdio
> > Running with RR_VALUE=record completes successfully.
> >
> > Running with RR_VALUE=replay caues exit with error:
> >
> > "qemu-system-x86_64: Missing character write event in the replay log"
> >
> > These problems are same with qemu 2.12 (both vanilla and with previous
> > versions of these patches applied). Furthemore, I consider whole
> > icount mode broken and determinism isn't achievable.
> > The irony is that I actually don't need record/replay feature. I've
> > tried to use it only as instrument to debug failing determinism in
> > qemu code. But since replay/record feature itself relies on
> > determinism, which is broken, it's no wonder why it fails also (I just
> > hoped to bypass it).
> >
> > Contact me if you need more details. I just tired a lot trying to get
> > all these things working... Hope is leaving me...
>
> Can you share the kernel in case the icount still broken?
>
>
> Pavel Dovgalyuk
>
> --

С уважением,
  Артем Писаренко