linux-kernel - Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC

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Message-Id: <905DFDCE-71A5-4711-A31B-9FCFEA2CFC52@amacapital.net>
Date:   Mon, 7 Dec 2020 08:53:59 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE



> On Dec 7, 2020, at 8:38 AM, Thomas Gleixner <tglx@...utronix.de> 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?