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Re: [xen-devel] System time monotonicity

To: "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [xen-devel] System time monotonicity
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 09 Apr 2008 15:41:43 +0100
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On 9/4/08 15:25, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Let me clarify... unless my reading of the code is wrong, ALL hvm
> guests that rely on ANY (virtual) platform timer are UNKNOWINGLY
> relying on the physical TSCs.  Thus if the underlying physical
> system has unsynchronized TSCs, different vcpus in an SMP HVM
> guest (or even the SAME vcpu when rescheduled on another pcpu)
> may find that consecutive reads of ANY (virtual) platform timer
> are unexpectedly non-monotonic, which violates the whole purpose
> of using a PLATFORM timer.

This is all true. The logic in vpt.c should be fixed to use Xen's concept of
system time and everything, guest TSC included, should be derived from that.

 -- Keir

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