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xen-devel
[Xen-devel] Re: pvclock in userland (reprise)
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To: |
"Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: pvclock in userland (reprise) |
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From: |
Keir Fraser <keir.fraser@xxxxxxxxxxxxx> |
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Date: |
Thu, 17 Sep 2009 20:23:14 +0100 |
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Cc: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx> |
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Thu, 17 Sep 2009 12:23:41 -0700 |
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Aco3wIEUOMEVM/j+SGeKFmFaWmbhWgACRF3UAAA5/QAAAHSniA== |
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Thread-topic: |
pvclock in userland (reprise) |
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Microsoft-Entourage/12.20.0.090605 |
On 17/09/2009 20:13, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx> wrote:
>>> A remaining hard problem is that this single
>>> "userland-accessible shared page" must be somehow
>>> made available to apps (I suggested a rdmsr emulated
>>> by Xen so that it works in userland) and must be
>>> mapped into the app address space without kernel
>>> changes. I think someone (Keir?) suggested this
>>> problem was solveable before we got sidetracked
>>> on the need-vcpu-number-in-userland problem.
>> I don't think mapping things into application address space is really
>> possible without guest kernel changes. The guest kernel owns and manages
>> the pte that you'd be overwriting. Just blatting the pte would not be
>> good form.
>
> Maybe you can write a device driver in the guest that sets up mapping against
> the (virtual) physical memory, then use mmap() in the app?
Yeah, that'd work. Doesn't even really need any Xen changes -- the
stable-TSC flag is in CPUID already, and the guest driver can concoct a
pvclock page I would think (e.g., from its own Xen pvclock TSC scaling
parameters).
-- Keir
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