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xen-devel
Re: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
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To: |
Keir Fraser <keir.fraser@xxxxxxxxxxxxx> |
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Subject: |
Re: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL |
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From: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
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Date: |
Tue, 04 Aug 2009 17:05:49 -0700 |
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Cc: |
Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx> |
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Tue, 04 Aug 2009 17:06:12 -0700 |
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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 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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