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

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Message-ID: <20201208181107.GA31442@fuller.cnet>
Date:   Tue, 8 Dec 2020 15:11:07 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
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>,
        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 Tue, Dec 08, 2020 at 05:02:07PM +0100, Thomas Gleixner wrote:
> On Tue, Dec 08 2020 at 16:50, Maxim Levitsky wrote:
> > On Mon, 2020-12-07 at 20:29 -0300, Marcelo Tosatti wrote:
> >> > +This ioctl allows to reconstruct the guest's IA32_TSC and TSC_ADJUST value
> >> > +from the state obtained in the past by KVM_GET_TSC_STATE on the same vCPU.
> >> > +
> >> > +If 'KVM_TSC_STATE_TIMESTAMP_VALID' is set in flags,
> >> > +KVM will adjust the guest TSC value by the time that passed since the moment
> >> > +CLOCK_REALTIME timestamp was saved in the struct and current value of
> >> > +CLOCK_REALTIME, and set the guest's TSC to the new value.
> >> 
> >> This introduces the wraparound bug in Linux timekeeping, doesnt it?
> 
> Which bug?

max_cycles overflow. Sent a message to Maxim describing it.

> 
> > It does.
> > Could you prepare a reproducer for this bug so I get a better idea about
> > what are you talking about?
> >
> > I assume you need very long (like days worth) jump to trigger this bug
> > and for such case we can either work around it in qemu / kernel 
> > or fix it in the guest kernel and I strongly prefer the latter.
> >
> > Thomas, what do you think about it?
> 
> For one I have no idea which bug you are talking about and if the bug is
> caused by the VMM then why would you "fix" it in the guest kernel.

1) Stop guest, save TSC value of cpu-0 = V.
2) Wait for some amount of time = W.
3) Start guest, load TSC value with V+W.

Can cause an overflow on Linux timekeeping.

> Aside of that I think I made it pretty clear what the right thing to do
> is.

Sure: the notion of a "unique TSC offset" already exists (it is detected
by write TSC logic, and not explicit in the interface, though).

But AFAIK it works pretty well.

Exposing a single TSC value on the interface level seems alright to
me...