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[Qemu-devel] [Bug 1797033] Re: Running with -rtc clock=vm, base=<datetim
From: |
Artem Pisarenko |
Subject: |
[Qemu-devel] [Bug 1797033] Re: Running with -rtc clock=vm, base=<datetime> introduces arbitrary base shift at guest startup |
Date: |
Wed, 10 Oct 2018 05:39:49 -0000 |
Why I didn't posted patch to qemu-devel ?
I have no idea how to patch it correctly, because it isn't clear how these
things are expected to work when referenced across all qemu code and different
use cases. Should vl.c/qemu_get_timedate() just be fixed ? Does each caller
expect same behavior from it? Or only selected hw/* RTC implementations (such
as hw/timer/mc146818rtc.c) have to be modified to use alternative function ? Or
maybe it isn't specific to clock=vm and should be considered bad behavior in
any case (i.e. time shouldn't advance before guest execution being started)?
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https://bugs.launchpad.net/bugs/1797033
Title:
Running with -rtc clock=vm,base=<datetime> introduces arbitrary base
shift at guest startup
Status in QEMU:
New
Bug description:
When specifying 'base' for RTC to start with, it has incorrect
implementation in combination with clock=vm.
I inspected source code. This is because it uses host clock
(qemu_time() function return value) as reference with
'rtc_date_offset' operations across several places in code before
guest execution starts, which has no relevance with clock=vm.
It works in vast majority of cases only thanks to combination of some
luck and fast execution of qemu initialization phase relative to host
real time (i.e. multiple calls of qemu_time() returns same value). But
if qemu execution is being slow down by high CPU load on host or
started just before value of second changes, it may accumulate at
least 1 second (and in hard circumstances even 2+ seconds) of delay
from specified base datetime before guest becomes ready to start.
This behavior breaks determinism of guest execution in icount mode.
(Even if guest doesn't cares about precise time, just one second shift
may trigger such chain of changes which accumulates significant
difference in guest state at the moment when guest OS finishes
booting.)
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