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

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Message-ID: <b6e0656b-4e3f-cf47-5ec9-eead44b2f2e9@redhat.com>
Date:   Thu, 10 Dec 2020 12:42:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.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>,
        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>,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On 07/12/20 18:41, Thomas Gleixner wrote:
> Right this happens still occasionally, but for quite some time this is
> 100% firmware sillyness and not a fundamental property of the hardware
> anymore.

It's still a fundamental property of old hardware.  Last time I tried to 
kill support for processors earlier than Core 2, I had to revert it. 
That's older than Nehalem.

>> We try to catch such situation in KVM instead of blowing up but
>> this may still result in subtle bugs I believe. Maybe we would be better
>> off killing all VMs in case TSC ever gets unsynced (by default).
> 
> I just ran a guest on an old machine with unsynchronized TSCs and was
> able to observe clock monotonic going backwards between two threads
> pinned on two vCPUs, which _is_ bad. Getting unsynced clocks reliably
> under control is extremly hard.

Using kvmclock?  (Half serious: perhaps a good reason to have per-vCPU 
offsets is to be able to test what happens with unsynchronized TSCs...).

Paolo