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[Qemu-devel] [Bug 1790460] Re: -icount, sleep=off mode is broken (target

From: Artem Pisarenko
Subject: [Qemu-devel] [Bug 1790460] Re: -icount, sleep=off mode is broken (target slows down or hangs)
Date: Tue, 04 Sep 2018 12:10:40 -0000

This modification also fixes issue:
Although I tested it only on v2.12.0 and didn't noticed performance improvement.

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  -icount,sleep=off mode is broken (target slows down or hangs)

Status in QEMU:

Bug description:
  QEMU running with options "-icount,sleep=off -rtc clock=vm" doesn't execute 
emulation at maximum possible speed.
  Target virtual clock may run faster or slower than realtime clock by N times, 
where N value depends on various unrelated conditions (i.e. random from the 
user point of view). The worst case is when target hangs (hopefully, early in 
booting stage).
  Example scenarios I've described here: 

  QEMU process just sleeps most of the time (polling, waiting some
  condition, etc.). I've tried to debug issue and came to 99% conclusion
  that there are racing somewhere in qemu internals.

  The feature is broken since v2.6.0 release.
  Bad commit is 281b2201e4e18d5b9a26e1e8d81b62b5581a13be by Pavel Dovgalyuk, 
03/10/2016 05:56 PM:

    icount: remove obsolete warp call
    qemu_clock_warp call in qemu_tcg_wait_io_event function is not needed
    anymore, because it is called in every iteration of main_loop_wait.
    Reviewed-by: Paolo Bonzini <address@hidden>

    Signed-off-by: Pavel Dovgalyuk <address@hidden>
    Message-Id: <address@hidden>
    Signed-off-by: Paolo Bonzini <address@hidden>

  I've reverted commit to all major releases and latest git master
  branch. Issue was fixed for all of them. My adaptation is trivial:
  just restoring removed function call before "qemu_cond_wait(...)"

  I'm sure following bugs are just particular cases of the issue:
  #1774677, #1653063 .

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