[Qemu-devel] [PATCH 0/3] Introduce attributes for timers subsystem and remove QEMU_CLOCK_VIRTUAL_EXT clock type

[Artem Pisarenko]

Recent patches from series [PATCH v6] "Fixing record/replay and adding reverse 
debugging" introduced new clock type QEMU_CLOCK_VIRTUAL_EXT and replaced 
virtual timers in some external subsystems with it.
This resulted in small change to existing behavior, which I consider to be 
unacceptable.
Processing of virtual timers, belonging to new clock type, was kicked off to 
main loop, which made them asynchronous with vCPU thread and, in icount mode, 
with whole guest execution. This breaks expected determinism in 
non-record/replay icount mode of emulation where these "external subsystems" 
are isolated from host (i.e. they external only to guest core, not to emulation 
environment).

Example for slirp ("user" backend for network device):
User runs qemu in icount mode with rtc clock=vm without any external 
communication interfaces but with "-netdev user,restrict=on". It expects 
deterministic execution, because network services are emulated inside qemu and 
isolated from host. There are no reasons to get reply from DHCP server with 
different delay or something like that.

These series of patches revert those commits and reimplement their 
modifications in a better way.

Current implementation of timers/clock processing is confusing (at least for 
me) because of exceptions from design concept behind them, introduced by icount 
mode (which adds QEMU_CLOCK_VIRTUAL_RT). Adding QEMU_CLOCK_VIRTUAL_EXT just 
made things even more complicated. I consider these "alternative" virtual 
clocks to be some kind of hacks being convinient only to authors of relevant 
qemu features. Lets don't touch fundamental clock types and keep them 
orthogonal to special cases of timers handling.

As far as I understand, original intention of author was just to make 
optimization in replay log by avoiding storing extra events which don't change 
guest state directly. Indeed, for example, ipv6 icmp timer in slirp does things 
which external to guest core and ends with sending network packet to guest, but 
record/replay will anyway catch event, corresponding to packet reception in 
guest network frontend, and store it to replay log, so there are no need in 
making checkpoint for corresponding clock when that timer fires.
If so, then we just need to skip checkpoints for clock values, when only these 
specific timers are run. It is individual timers which are specific, not clock.
Adding some kind of attribute/flag/property to individual timer allows any 
special qemu feature (such as record/replay) to inspect it in any place of code 
and handle as needed. This design acheives less dependencies, more transparency 
and has more intuitive and clear sense. For record/replay feature it's acheived 
with 'EXTERNAL' attribute. The only drawback is that it required to add extra 
cycle of traversal through active timer list in timerlist_run_timers() function 
(see patch 3), but I've optimized it in a way that performance degradation 
expected to be very small and appear only in record/replay mode.

P.S. I've tried to test record/replay with slirp, but in replay mode qemu 
stucks at guest linux boot after "Configuring network interfaces..." message, 
where DHCP communication takes place. It's broken in a same way both in master 
and master with reverted commits being fixed.


Artem Pisarenko (3):
  Revert some patches from recent series [PATCH v6] "Fixing
    record/replay and adding reverse debugging", which introduced new
    virtual clock type for use in external subsystems.     These changes
    breaks desired behavior in non-record/replay usage scenarios.
  Introduce attributes to qemu timer subsystem
  Restores record/replay behavior related to special virtual clock
    processing for timers used in external subsystems.

 include/block/aio.h         |  50 +++++++++++++++++-
 include/qemu/coroutine.h    |   5 +-
 include/qemu/timer.h        | 125 ++++++++++++++++++++++++++++++++++++--------
 slirp/ip6_icmp.c            |   9 ++--
 tests/ptimer-test-stubs.c   |   7 +--
 ui/input.c                  |   9 ++--
 util/qemu-coroutine-sleep.c |   6 ++-
 util/qemu-timer.c           |  81 +++++++++++++++++++---------
 8 files changed, 227 insertions(+), 65 deletions(-)

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2.7.4