From: Avi Kivity <avi@redhat.com>
To: Zachary Amsden <zamsden@redhat.com>
Cc: Julien Desfossez <ju@klipix.org>, kvm@vger.kernel.org
Subject: Re: vread in kvm_clock
Date: Sun, 19 Dec 2010 17:27:21 +0200 [thread overview]
Message-ID: <4D0E2459.7060907@redhat.com> (raw)
In-Reply-To: <4D0BA159.5050400@redhat.com>
On 12/17/2010 07:43 PM, Zachary Amsden wrote:
> On 12/15/2010 10:16 AM, Julien Desfossez wrote:
>> Hi,
>>
>> I'm currently working with the kvm clocksource and I'm wondering if
>> we could implement the vread function for this clock source when we
>> are running on a host with constant_tsc.
>> If I understand correctly the hv_clock structure is per_cpu because
>> of the eventual frequency changes, but in the case of constant_tsc
>> (and after validation that the TSC is synchronized across all the
>> cores) I think we could have a working vread function.
>>
>> In case of migration, could we have a fallback in case we detect we
>> end up on a CPU without constant_tsc ?
>>
>> Any advice/explanation would be greatly appreciated !
>
> It's a bit more complex than that. In addition to the problem you
> mention with migration, even if the TSC is synchronized, the kvmclock
> still is not, even with constant_tsc. There is measurement error in
> between reading the TSC and computing the per_cpu hv_clock offset
> which varies between CPUs.
>
What about using rdtscp?
We could also disable kvmclock if constant_tsc and migration is not
desired, or if constant_tsc and the new tsc multiplier on bulldozers is
available on all machines in the migration cluster.
--
error compiling committee.c: too many arguments to function
next prev parent reply other threads:[~2010-12-19 15:27 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-15 20:16 vread in kvm_clock Julien Desfossez
2010-12-17 17:43 ` Zachary Amsden
2010-12-19 15:27 ` Avi Kivity [this message]
2010-12-20 1:15 ` Zachary Amsden
2010-12-20 6:16 ` Avi Kivity
2010-12-20 20:48 ` Zachary Amsden
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