Virtualization - Apr 2022 - [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as a perf event clock

On Tue, Apr 26 2022 at 09:51, Adrian Hunter wrote:
> On 25/04/22 20:05, Thomas Gleixner wrote:
>> On Mon, Apr 25 2022 at 16:15, Adrian Hunter wrote:
>>> On 25/04/22 12:32, Thomas Gleixner wrote:
>>>> It's hillarious, that we still cling to this pvclock
abomination, while
>>>> we happily expose TSC deadline timer to the guest. TSC virt
scaling was
>>>> implemented in hardware for a reason.
>>>
>>> So you are talking about changing VMX TCS Offset on every VM-Entry
to try to hide
>>> the time jumps when the VM is scheduled out?  Or neglect that and
just let the time
>>> jumps happen?
>>>
>>> If changing VMX TCS Offset, how can TSC be kept consistent between
each VCPU i.e.
>>> wouldn't that mean each VCPU has to have the same VMX TSC
Offset?
>> 
>> Obviously so. That's the only thing which makes sense, no?
>
> [ Sending this again, because I notice I messed up the email
"From" ]
>
> But wouldn't that mean changing all the VCPUs VMX TSC Offset at the
same time,
> which means when none are currently executing?  How could that be done?
Why would you change TSC offset after the point where a VM is started
and why would it be different per vCPU?

Time is global and time moves on when a vCPU is scheduled out. Anything
else is bonkers, really. If the hypervisor tries to screw with that then
how does the guest do timekeeping in a consistent way?

    CLOCK_REALTIME = CLOCK_MONOTONIC + offset

That offset changes when something sets the clock, i.e. clock_settime(),
settimeofday() or adjtimex() in case that NTP cannot compensate or for
the beloved leap seconds adjustment. At any other time the offset is
constant.

CLOCK_MONOTONIC is derived from the underlying clocksource which is
expected to increment with constant frequency and that has to be
consistent accross _all_ vCPUs of a particular VM.

So how would a hypervisor 'hide' scheduled out time w/o screwing up
timekeeping completely?

The guest TSC which is based on the host TSC is:

    guestTSC = offset + hostTSC * factor;

If you make offset different between guest vCPUs then timekeeping in the
guest is screwed.

The whole point of that paravirt clock was to handle migration between
hosts which did not have the VMCS TSC scaling/offset mechanism. The CPUs
which did not have that went EOL at least 10 years ago.

So what are you concerned about?

Thanks,

        tglx