Re: [PATCH 2/2] Remove -tdf

[Anthony Liguori <anthony@codemonkey.ws>]

Dor Laor wrote:
> Anthony Liguori wrote:
>> The last time I posted the KVM patch series to qemu-devel, the -tdf 
>> patch met with
>> some opposition.  Since today we implement timer catch-up in the 
>> in-kernel PIT and
>> the in-kernel PIT is used by default, it doesn't seem all that 
>> valuable to have
>> timer catch-up in userspace too.
>>
>> Removing it will reduce our divergence from QEMU.
>>
>>   
> IMHO the in kernel PIT should go away, there is no reason to keep it 
> except that userspace PIT drifts.

I agree fully :-)  But there's certainly no reason to keep -tdf and the 
in-kernel PIT.  Since we're using the in-kernel PIT right now, I'd like 
to get rid of -tdf.

> Currently both in-kernel PIT and even the in kernel irqchips are not 
> 100% bullet proof.
> Of course this code is a hack, Gleb Natapov has send better fix for 
> PIT/RTC to qemu list.
> Can you look into them:
> http://www.mail-archive.com/kvm@vger.kernel.org/msg01181.html

Paul Brook's initial feedback is still valid.  It causes quite a lot of 
churn and may not jive well with a virtual time base.  An advantage to 
the current -tdf patch is that it's more contained.  I don't think 
either approach is going to get past Paul in it's current form.

I'd still like to see some harder evidence of the benefits of tdf.  For 
a specific guest, with a specific configuration, how much better is the 
drift with this series.  The answer shouldn't be "movie's play better" :-)

Also, it's important that this is reproducible in upstream QEMU and not 
just in KVM.  If we can make a compelling case for the importance of 
this, we can possibly work out a compromise.

Regards,

Anthony Liguori