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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Marcelo Tosatti <mtosatti@redhat.com>,
        Maxim Levitsky <mlevitsk@redhat.com>
Cc:     kvm@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jim Mattson <jmattson@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        open list <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "maintainer\:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" <x86@kernel.org>,
        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        Shuah Khan <shuah@kernel.org>,
        Andrew Jones <drjones@redhat.com>,
        Oliver Upton <oupton@google.com>,
        "open list\:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
In-Reply-To: <20201208181232.GB31442@fuller.cnet>
References: <20201203171118.372391-1-mlevitsk@redhat.com> <20201203171118.372391-2-mlevitsk@redhat.com> <20201207232920.GD27492@fuller.cnet> <05aaabedd4aac7d3bce81d338988108885a19d29.camel@redhat.com> <87sg8g2sn4.fsf@nanos.tec.linutronix.de> <6f64558a029574444da417754786f711c2fec407.camel@redhat.com> <20201208181232.GB31442@fuller.cnet>
Date:   Tue, 08 Dec 2020 22:35:27 +0100
Message-ID: <87360g2d7k.fsf@nanos.tec.linutronix.de>
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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