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

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Message-ID: <87sg8g2sn4.fsf@nanos.tec.linutronix.de>
Date:   Tue, 08 Dec 2020 17:02:07 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     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 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?

> 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.

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

Thanks,

        tglx