Re: [Xen-devel] [PATCH 13/13] tools/libxl: Add 'vtsc_khz' option to set guest TSC rate

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/09/15 14:53, Haozhong Zhang wrote:
> On Tue, Sep 29, 2015 at 11:28:38AM +0100, Ian Campbell wrote:
>> On Tue, 2015-09-29 at 11:24 +0100, Andrew Cooper wrote:
>>> On 29/09/15 11:13, Haozhong Zhang wrote:
>>>> On Tue, Sep 29, 2015 at 11:04:14AM +0100, Ian Campbell wrote:
>>>>> On Mon, 2015-09-28 at 15:13 +0800, Haozhong Zhang wrote:
>>>>>> This patch adds an option 'vtsc_khz' to allow users to set vcpu's
>>>>>> TSC
>>>>>> rate in KHz. In the case that tsc_mode = 'default', the default
>>>>>> value of
>>>>>> 'vtsc_khz' option is the host TSC rate which is used when
>>>>>> 'vtsc_khz'
>>>>>> option is set to 0 or does not appear in the configuration. In all
>>>>>> other
>>>>>> cases of tsc_mode, 'vtsc_khz' option is just ignored.
>>>>>>
>>>>>> Another purpose of adding this option is to keep vcpu's TSC rate
>>>>>> across
>>>>>> guest reboot. In existing code, a new domain is created from the
>>>>>> configuration of the previous domain which was just rebooted.
>>>>>> vcpu's TSC
>>>>>> rate is not stored in the configuration and the host TSC rate is
>>>>>> the
>>>>>> used as vcpu's TSC rate. This works fine unless the previous domain
>>>>>> was
>>>>>> migrated from another host machine with a different host TSC rate
>>>>>> than
>>>>>> the current one.
>>>>> I understand why this is necessary over a migration, but why is it
>>>>> important to be able to retain the TSC rate across a reboot? What is
>>>>> the
>>>>> usecase there?
>>>>>
>>>> No usecase so far. Is 'making a virtual machine more like a physical
>>>> machine' reasonable here? (I suppose a physical machine retains TSC
>>>> rate across reboot)
>>> There are situations such as altering firmware settings which can cause
>>> the TSC rate to differ across reboot.  I don't see any reason to try and
>>> maintain it across VM reboots.
>> Right. If it happens to come for free as a side effect of making it work
>> for migration then fine.
>>
>> But it seems to me that tsc rate could/should be in the hypervisors save
>> blob and require no interaction with the toolstack once it is latched when
>> the domain is built.
>>
>> Ian.
>>
> Seemingly I don't need vtsc_khz at all if retaining tsc rate across
> reboot is unnecessary (or even wrong). The migration of tsc rate is
> already done through write_tsc_info() and handle_tsc_info() w/o this
> patch. I'll check if "xl save/restore" also go through this path. If
> so, I think vtsc_khz can be removed.

I can confirm from my rewrite of migration that tsc info is explicitly
saved and restored in both PV and HVM migration.

~Andrew

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