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) + offsetThe 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
- 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.
- (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