Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Andy Lutomirski]

> On Dec 7, 2020, at 9:00 AM, Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
> 
> On Mon, 2020-12-07 at 08:53 -0800, Andy Lutomirski wrote:
>>>> On Dec 7, 2020, at 8:38 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>>> 
>>> On Mon, Dec 07 2020 at 14:16, Maxim Levitsky wrote:
>>>>> On Sun, 2020-12-06 at 17:19 +0100, Thomas Gleixner wrote:
>>>>> From a timekeeping POV and the guests expectation of TSC this is
>>>>> fundamentally wrong:
>>>>> 
>>>>>     tscguest = scaled(hosttsc) + offset
>>>>> 
>>>>> The TSC has to be viewed systemwide and not per CPU. It's systemwide
>>>>> used for timekeeping and for that to work it has to be synchronized. 
>>>>> 
>>>>> Why would this be different on virt? Just because it's virt or what? 
>>>>> 
>>>>> Migration is a guest wide thing and you're not migrating single vCPUs.
>>>>> 
>>>>> This hackery just papers over he underlying design fail that KVM looks
>>>>> at the TSC per vCPU which is the root cause and that needs to be fixed.
>>>> 
>>>> I don't disagree with you.
>>>> As far as I know the main reasons that kvm tracks TSC per guest are
>>>> 
>>>> 1. cases when host tsc is not stable 
>>>> (hopefully rare now, and I don't mind making
>>>> the new API just refuse to work when this is detected, and revert to old way
>>>> of doing things).
>>> 
>>> That's a trainwreck to begin with and I really would just not support it
>>> for anything new which aims to be more precise and correct.  TSC has
>>> become pretty reliable over the years.
>>> 
>>>> 2. (theoretical) ability of the guest to introduce per core tsc offfset
>>>> by either using TSC_ADJUST (for which I got recently an idea to stop
>>>> advertising this feature to the guest), or writing TSC directly which
>>>> is allowed by Intel's PRM:
>>> 
>>> For anything halfways modern the write to TSC is reflected in TSC_ADJUST
>>> which means you get the precise offset.
>>> 
>>> The general principle still applies from a system POV.
>>> 
>>>    TSC base (systemwide view) - The sane case
>>> 
>>>    TSC CPU  = TSC base + TSC_ADJUST
>>> 
>>> The guest TSC base is a per guest constant offset to the host TSC.
>>> 
>>>    TSC guest base = TSC host base + guest base offset
>>> 
>>> If the guest want's this different per vCPU by writing to the MSR or to
>>> TSC_ADJUST then you still can have a per vCPU offset in TSC_ADJUST which
>>> is the offset to the TSC base of the guest.
>> 
>> How about, if the guest wants to write TSC_ADJUST, it can turn off all paravirt features and keep both pieces?
>> 
> 
> This is one of the things I had in mind recently.
> 
> Even better, we can stop advertising TSC_ADJUST in CPUID to the guest 
> and forbid it from writing it at all.

Seems reasonable to me.

It also seems okay for some MSRs to stop working after the guest enabled new PV timekeeping.

I do have a feature request, though: IMO it would be quite nifty if the new kvmclock structure could also expose NTP corrections. In other words, if you could expose enough info to calculate CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC, and CLOCK_REALTIME, then we could have paravirt NTP.

Bonus points if whatever you do for CLOCK_REALTIME also exposes leap seconds in a race free way :). But I suppose that just exposing TAI and letting the guest deal with the TAI - UTC offset itself would get the job done just fine.