Re: [Xen-devel] Xen timing mode

[Priya <pbhat@xxxxxxxxxxxx>]

The Xen Interface manual for Xen v2.0 (attached) Page 4 defines Domain - virtual time as

" The time that progresses at the same pace as system time, but only
while a domain is executing -- it stops while a domain is de-scheduled.
Therefore the share of the CPU that a domain receives is indicated by
the rate at which its virtual time increases."

I am interesting in finding out if any of the timing system
calls/instructions like gettimeofday(), hwclock(), rdtsc() can read
this domain virtual time.

Thanks, again !

PS: I could not find a similar documentation for Xen v3.0 or later

On Tue, Feb 23, 2010 at 12:18 PM, Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:

What do you mean by domain - virtual time?

On 23/02/2010 17:12, "Priya" <pbhat@xxxxxxxxxxxx> wrote:

> Thanks !
>
> In that case, how does a para-virtualized user domain find the domain -
> virtual time ?
>
> --Priya
>
> On Tue, Feb 23, 2010 at 12:08 PM, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
> wrote:
>> Independent_wallclock is specific to Linux PV guests, and is set within the
>> guest. Timer_mode is specific to HVM guests and is a domain build-time
>> config option.
>>
>>  -- Keir
>>
>> On 23/02/2010 16:50, "PBhat" <pbhat@xxxxxxxxxxxx> wrote:
>>
>>>
>>> A question about timing modes in Xen came up after reading the documentation
>>> on
>>> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(int
>>> er

>>> <http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28

>>> inter>
>>> rupts)|(timer)
>>> configuration file options  and I was wondering if you could help me with
>>> it.
>>>
>>>     * I know that Xen has a facility on the user domains to change the
>>> timing from something called the ' http://openskill.info/infobox.php?ID=1451
>>> independent wallclock ' to 'dependent clock' and vice versa. The way to do
>>> this is to toggle the parameter /proc/sys/xen/independent_wallclock between
>>> 0 (dependent) and 1 (independent)
>>>
>>>
>>>     * Now, the documentation on the configuration options for xen virtual
>>> machine under the heading of Timers contains no mention of the above
>>> mentioned facility, but instead talks about something called a 'timer_mode'
>>>
>>>
>>>  timer_mode: Timer mode (0=delay virtual time when ticks are missed;
>>> 1=virtual time is always wallclock time
>>>       timer_mode (default=1; Value='TIMER_MODE')
>>>
>>> Now my question is whether the timer mode  = 1 is equivalent to independent_
>>> wallclock?
>>>
>>> I think the answer is no. While timer_mode = 1 decides whether the virtual
>>> machine is able to read the Domain-0 time, the independent_wallclock decides
>>> whether you need NTP sychronization on both (user and control) domains or
>>> not.
>>>
>>> Namely, in the timer_mode = 1, the gettimeofday() called from the virtual
>>> machine will return the domain virtual time, but time_mode = 0
>>> gettimeofday() instruction from the virtual machine will return the system
>>> time.
>>>
>>> The independent and dependent modes have an effect on whether the virtual
>>> machines can use the NTP or any other synchronization of Domain-0.
>>>
>>> Is my understanding correct?
>>>
>>> I further want to know whether the working remains similar on Xen HVMs also.
>>> In the sense that are Hardware based virtual machines capable of reading the
>>> Domain-0 system time in an analogous way?
>>>
>>>
>>> Your help would be much appreciated. I am new to this area and I apologize
>>> if my questions seem too basic.
>>
>>
>>
>
>

Attachment: xen_interface.pdf
Description: Adobe PDF document

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