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RE: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
From: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Date: Wed, 5 Aug 2009 13:35:45 +0800
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Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>, Levon <levon@xxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, John, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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Thread-topic: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
>From: Jeremy Fitzhardinge
>Sent: 2009年8月5日 8:06
>
>On 07/24/09 01:04, Keir Fraser wrote:
>> Okay, so the issue you are worried about is not specific to 
>Xen. So how is
>> native Linux tackling this, for example?
>>   
>
>Linux will use the tsc where possible, but regularly assesses its
>perceived accuracy and will move to a different clocksource if the tsc
>appears to the playing up.  I don't think it ever assumes the tsc is
>synced between CPU/cores.

It cares. See tsc_sync.c under x86 arch, where unsynced warps
mark tsc as unstable. 

Thanks,
Kevin

>
>It allows rdtsc from usermode, but it is generally considered 
>to be very
>buggy and ill-defined behaviour.  It makes no attempt to make usermode
>rdtsc in any way meaningful.  The exception is the vgettimeofday
>vsyscall which does Xen-like timekeeping, in which it gets the tsc,cpu
>tuple atomically, then scales it with timing parameters from 
>the kernel.
>
>    J
>
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