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

[Laurent Vivier <lvivier@redhat.com>]

On 13/12/2017 20:59, Alexander Graf wrote:
> 
> 
> On 13.12.17 20:29, Laurent Vivier wrote:
>> On 13/12/2017 20:19, Alexander Graf wrote:
>>>
>>>
>>> On 02.02.17 06:14, David Gibson wrote:
>>>> From: Laurent Vivier <lvivier@redhat.com>
>>>>
>>>> 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 <lvivier@redhat.com>
>>>> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
>>>
>>> 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 ;).
>>
>> Don't know if it can be related, but for migration we need:
> 
> 
> As expected, this did not fix it. I'll keep digging.
> 
> My hunch is that we now set VTB on different cores at different times,
> introducing tiny VTB offsets which can lead to negative TB differences
> inside the guest.

Did you find where is the problem?

Can I help?

Thanks,
Laurent