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

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Message-ID: <87360g2d7k.fsf@nanos.tec.linutronix.de>
Date:   Tue, 08 Dec 2020 22:35:27 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Maxim Levitsky <mlevitsk@...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 15:12, Marcelo Tosatti wrote:
> On Tue, Dec 08, 2020 at 06:25:13PM +0200, Maxim Levitsky wrote:
>> On Tue, 2020-12-08 at 17:02 +0100, Thomas Gleixner wrote:
>> The "bug" is that if VMM moves a hardware time counter (tsc or anything else) 
>> forward by large enough value in one go, 
>> then the guest kernel will supposingly have an overflow in the time code.
>> I don't consider this to be a buggy VMM behavior, but rather a kernel
>> bug that should be fixed (if this bug actually exists)
>
> It exists.

In the VMM. 

>> We are talking about the fact that TSC can jump forward by arbitrary large
>> value if the migration took arbitrary amount of time, which 
>> (assuming that the bug is real) can crash the guest kernel.
>
> QE reproduced it.

Sure, that's what QE is about. Just your conclusion is wrong.

Thanks,

        tglx