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Re: [Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 02 Sep 2009 14:44:47 -0700
Cc: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
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On 09/02/09 00:20, Keir Fraser wrote:
> The problem is a bit easier with vsyscall potentially. For example, give
> each thread its own vsyscall clock data area (easy?), updated by kernel
> whenever the thread is scheduled, and increment a version counter, checked
> before and after by the vsyscall operation.
>   

Yes.  Perhaps the very simplest way would be to make the kernel update
the pvclock version counter on context switch, the same way Xen does;
that would allow the usermode vsyscall code to use exactly the same
algorithm as the kernel code.  Would Xen cope with that?

> Well, I don't know how easy or fast that could actually be implemented, but
> I'm at least confident it could work. But it does need kernel assistance.
>   

Yes.  I'm very uneasy about letting usermode have direct access to bits
of Xen without the kernel's knowledge anyway.  It suddenly means we need
to not only maintain a Xen<->kernel ABI, but a Xen<->usermode ABI as well.

    J

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