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Re: [Qemu-ppc] [PULL 095/107] spapr: clock should count only if vm is ru

From: Alexander Graf
Subject: Re: [Qemu-ppc] [PULL 095/107] spapr: clock should count only if vm is running
Date: Wed, 13 Dec 2017 20:19:52 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 02.02.17 06:14, David Gibson wrote:
> From: Laurent Vivier <address@hidden>
> This is a port to ppc of the i386 commit:
>     00f4d64 kvmclock: clock should count only if vm is running
> We remove timebase_post_load function, and use the VM state
> change handler to save and restore the guest_timebase (on stop
> and continue).
> We keep timebase_pre_save to reduce the clock difference on
> migration like in:
>     6053a86 kvmclock: reduce kvmclock difference on migration
> Time base offset has originally been introduced by commit
>     98a8b52 spapr: Add support for time base offset migration
> So while VM is paused, the time is stopped. This allows to have
> the same result with date (based on Time Base Register) and
> hwclock (based on "get-time-of-day" RTAS call).
> Moreover in TCG mode, the Time Base is always paused, so this
> patch also adjust the behavior between TCG and KVM.
> VM state field "time_of_the_day_ns" is now useless but we keep
> it to be able to migrate to older version of the machine.
> As vmstate_ppc_timebase structure (with timebase_pre_save() and
> timebase_post_load() functions) was only used by vmstate_spapr,
> we register the VM state change handler only in ppc_spapr_init().
> Signed-off-by: Laurent Vivier <address@hidden>
> Signed-off-by: David Gibson <address@hidden>

Just a small heads-up: I've been debugging an OpenQA regression lately
where our automated testing regressed with QEMU 2.9. With stock 2.9.1, I
get a failure rate of "weird" effects (probably TB divergence between
vcpus) of ~30%. With this patch reverted it's back to 0%.

I *think* something here causes the TB offset of multiple threads (I'm
running -smp 2,threads=2) to diverge.

I'll keep debugging things tomorrow, but I'll be happy to see anyone
else beat me to analyze what is going wrong ;).


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