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Re: [Xen-devel] Time went backwards

To: "Langsdorf, Mark" <mark.langsdorf@xxxxxxx>, Niraj Tolia <ntolia@xxxxxxxxx>, Xen Developers <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Time went backwards
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Sat, 16 Feb 2008 08:08:04 +0000
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On 15/2/08 18:04, "Langsdorf, Mark" <mark.langsdorf@xxxxxxx> wrote:

> He's got a dual-core system, but the logical grouping of the
> cores doesn't match the physical grouping.  Something like
> cores 0,1 share frequencies, but PowerNow! believes that
> cores 0 and 2 do.  So when a frequency change on core 0
> occurs, the Xen hypervisor gets alerted that cores 0 and 2
> have new frequencies.  This causes two errors with Xen -
> the HV believes core 1 has the old frequency, when it
> has the new frequency, and it believes core 2 has the new
> frequency, when it really has the old frequency.  Time errors
> will accumulate rapidly.
> Niraj -
> Please email me privately.  I have some more time to work on
> this issue and some new ideas on how to get information about it.

An easy way would be to allocate a new bit in a CPUID hypervisor leaf to
indicate that the PV guest can trust the core/thread information in the
usual CPUID leaves. Then we could populate the affinity maps in a way
similar to native Linux.

Or we could add a PV interface to obtain this information, which would be
less x86 specific.

 -- Keir

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