From mboxrd@z Thu Jan  1 00:00:00 1970
From: Keir Fraser <keir@xen.org>
Subject: Re: common time reference between domU and dom0
Date: Fri, 10 Dec 2010 10:03:39 +0000
Message-ID: <C927AB7B.C5A2%keir@xen.org>
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To: Henry Pepper <henrypepr@gmail.com>, xen-devel@lists.xensource.com
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All domains have access to Xen system time, either via hypercall or by
calculating it themselves directly given information Xen provides in shared
memory. Time interrupt/events can't really be missed - they are delivered
synchronously to the guest.

 -- Keir

On 10/12/2010 08:26, "Henry Pepper" <henrypepr@gmail.com> wrote:

> I was under the impression that none of the domains has the correct time.
>  Neither Dom0 nor DomU receives a timer interrupt, but only a timer
> event. Events that could be missed.
> So timing in the millisecond can have jitter and drift.
> Is there actually a way to keep a precise timer in a domain?
>=20
> On top of this I would think that both domains would have to have
> exclusive access to their own CPU in order to avoid being switched off
> of the CPU in periods of time(quantums of 30 ms using the default
> scheduler).
>=20
>   Henry
>=20
> On Fri, Dec 10, 2010 at 8:19 AM, Keir Fraser <keir@xen.org> wrote:
>> System time, as exposed to guests by Xen, should be good enough.
>>=20
>> =A0-- Keir
>>=20
>>=20
>> On 10/12/2010 05:23, "James Harper" <james.harper@bendigoit.com.au> wrot=
e:
>>=20
>>> I would like to be able to measure front/back ring message latency
>>> between a Windows DomU and Dom0. To do this I need a common time
>>> reference between the two, preferably with at least microsecond
>>> precision (latency <1us is irrelevant for my purposes). Does such a
>>> thing exist? A small amount of overhead in measuring this latency is
>>> acceptable.
>>>=20
>>> Thanks
>>>=20
>>> James
>>>=20
>>>=20
>>> _______________________________________________
>>> Xen-devel mailing list
>>> Xen-devel@lists.xensource.com
>>> http://lists.xensource.com/xen-devel
>>=20
>>=20
>>=20
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel
>>=20
>=20
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