Re: [PATCH 1/5] Add a global synchronization point for pvclock

From: Avi Kivity <avi@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Glauber Costa <glommer@redhat.com>,
	Jeremy Fitzhardinge <jeremy@goop.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Zachary Amsden <zamsden@redhat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock
Date: Tue, 20 Apr 2010 12:35:19 +0300	[thread overview]
Message-ID: <4BCD7557.9090502@redhat.com> (raw)
In-Reply-To: <20100420015733.GA28249@amt.cnet>

On 04/20/2010 04:57 AM, Marcelo Tosatti wrote:
>
>> Marcelo can probably confirm it, but he has a nehalem with an appearently
>> very good tsc source. Even this machine warps.
>>
>> It stops warping if we only write pvclock data structure once and forget it,
>> (which only updated tsc_timestamp once), according to him.
>>      
> Yes. So its not as if the guest visible TSCs go out of sync (they don't
> on this machine Glauber mentioned, or even on a multi-core Core 2 Duo),
> but the delta calculation is very hard (if not impossible) to get right.
>
> The timewarps i've seen were in the 0-200ns range, and very rare (once
> every 10 minutes or so).
>    

Might be due to NMIs or SMIs interrupting the rdtsc(); ktime_get() 
operation which establishes the timeline.  We could limit it by having a 
loop doing rdtsc(); ktime_get(); rdtsc(); and checking for some bound, 
but it isn't worthwhile (and will break nested virtualization for 
sure).  Better to have the option to calibrate kvmclock just once on 
machines with X86_FEATURE_NONSTOP_TRULY_RELIABLE _TSC_HONESTLY.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.