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.
TSC guest CPU = TSC guest base + CPU TSC_ADJUST
==>
TSC guest CPU = TSC host base + guest base offset + CPU TSC_ADJUST
The normal and sane case is just TSC_ADJUST == 0.
It's very cleanly decomposable.
Thanks,
tglx
