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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
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Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
In-Reply-To: <1dbbeefc7c76c259b55582468ccd3aab35a6de60.camel@redhat.com>
References: <20201203171118.372391-1-mlevitsk@redhat.com> <20201203171118.372391-2-mlevitsk@redhat.com> <87a6uq9abf.fsf@nanos.tec.linutronix.de> <1dbbeefc7c76c259b55582468ccd3aab35a6de60.camel@redhat.com>
Date:   Mon, 07 Dec 2020 17:38:36 +0100
Message-ID: <87a6up606r.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
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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