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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/09/15 11:28, 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.

There are a lot of blobs which fall into this category.  Others are
cpuid policy and guest-MSRs.  I have a longterm plan to fix them, which
is under very slow progress.

~Andrew