[Xen-devel] clock source in PV Linux

[Xen-devel] clock source in PV Linux

[Jan Beulich]

Jürgen, Boris,

I've noticed

<6>clocksource: Switched to clocksource tsc

as the final clocksource related boot message in a PV Dom0's
log with 5.4.2. Is it intentional that it's not the "xen" one
that gets used by default?

Thanks, Jan

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Re: [Xen-devel] clock source in PV Linux

[Jürgen Groß]

On 11.12.19 08:28, Jan Beulich wrote:
> Jürgen, Boris,
> 
> I've noticed
> 
> <6>clocksource: Switched to clocksource tsc
> 
> as the final clocksource related boot message in a PV Dom0's
> log with 5.4.2. Is it intentional that it's not the "xen" one
> that gets used by default?

I think this is fine. I just tested it and I'm seeing the same in dom0,
while in a PV domU "xen" is used per default.

In dom0 "tsc" should be okay in case it is stable. Or are you expecting
problems with that setting?


Juergen

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Re: [Xen-devel] clock source in PV Linux

[Jan Beulich]

On 11.12.2019 09:16, Jürgen Groß wrote:
> On 11.12.19 08:28, Jan Beulich wrote:
>> Jürgen, Boris,
>>
>> I've noticed
>>
>> <6>clocksource: Switched to clocksource tsc
>>
>> as the final clocksource related boot message in a PV Dom0's
>> log with 5.4.2. Is it intentional that it's not the "xen" one
>> that gets used by default?
> 
> I think this is fine. I just tested it and I'm seeing the same in dom0,
> while in a PV domU "xen" is used per default.
> 
> In dom0 "tsc" should be okay in case it is stable. Or are you expecting
> problems with that setting?

Well, first of all I found this surprising. Whether there are problems to
be expected largely depends on the reliability of the "stable" detection
in PV Dom0.

Jan

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Re: [Xen-devel] clock source in PV Linux

[Roman Shaposhnik]

On Wed, Dec 11, 2019 at 12:41 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 11.12.2019 09:16, Jürgen Groß wrote:
> > On 11.12.19 08:28, Jan Beulich wrote:
> >> Jürgen, Boris,
> >>
> >> I've noticed
> >>
> >> <6>clocksource: Switched to clocksource tsc
> >>
> >> as the final clocksource related boot message in a PV Dom0's
> >> log with 5.4.2. Is it intentional that it's not the "xen" one
> >> that gets used by default?
> >
> > I think this is fine. I just tested it and I'm seeing the same in dom0,
> > while in a PV domU "xen" is used per default.
> >
> > In dom0 "tsc" should be okay in case it is stable. Or are you expecting
> > problems with that setting?
>
> Well, first of all I found this surprising. Whether there are problems to
> be expected largely depends on the reliability of the "stable" detection
> in PV Dom0.

Related question: does this mean that tsc is now default for PVH as well?

The reason I'm asking is because I'm still a bit worried about the
clock drift with tsc.

Thanks,
Roman.

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Re: [Xen-devel] clock source in PV Linux

[Boris Ostrovsky]

On 12/18/19 12:36 AM, Roman Shaposhnik wrote:
> On Wed, Dec 11, 2019 at 12:41 AM Jan Beulich <jbeulich@suse.com> wrote:
>> On 11.12.2019 09:16, Jürgen Groß wrote:
>>> On 11.12.19 08:28, Jan Beulich wrote:
>>>> Jürgen, Boris,
>>>>
>>>> I've noticed
>>>>
>>>> <6>clocksource: Switched to clocksource tsc
>>>>
>>>> as the final clocksource related boot message in a PV Dom0's
>>>> log with 5.4.2. Is it intentional that it's not the "xen" one
>>>> that gets used by default?
>>> I think this is fine. I just tested it and I'm seeing the same in dom0,
>>> while in a PV domU "xen" is used per default.
>>>
>>> In dom0 "tsc" should be okay in case it is stable. Or are you expecting
>>> problems with that setting?
>> Well, first of all I found this surprising. Whether there are problems to
>> be expected largely depends on the reliability of the "stable" detection
>> in PV Dom0.
> Related question: does this mean that tsc is now default for PVH as well?
>
> The reason I'm asking is because I'm still a bit worried about the
> clock drift with tsc.


dom0 will use TSC for either PV or PVH:

xen_time_init():
        /* As Dom0 is never moved, no penalty on using TSC there */
         if (xen_initial_domain())
                 xen_clocksource.rating = 275;

But as far as TSC stability I'd think it should be sufficiently checked 
by generic TSC init code?



-boris

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