linux-kernel - Re: [PATCH 0/2] RFC: Precise TSC migration

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Message-ID: <26444d65-083b-5df4-52c9-c1cfad556b10@redhat.com>
Date:   Mon, 30 Nov 2020 17:59:08 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>, Oliver Upton <oupton@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 0/2] RFC: Precise TSC migration

On 30/11/20 17:54, Andy Lutomirski wrote:
>> I*do think*  however that we should redefine KVM_CLOCK_TSC_STABLE
>> in the documentation to state that it only guarantees invariance if the guest
>> doesn't mess with its own TSC.
>>
>> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE
>> in the guest kernel, when kvm is detected to avoid the guest even from trying
>> to sync TSC on newly hotplugged vCPUs.
>>
>> (The guest doesn't end up touching TSC_ADJUST usually, but it still might
>> in some cases due to scheduling of guest vCPUs)
>>
>> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug,
>> and TSC clocksource watchdog, and the later we might want to keep).
> If you're going to change the guest behavior to be more trusting of
> the host, I think
> the host should probably signal this to the guest using a new bit.
> 

Yes, a new CPUID bit takes longer to propagate to existing setups, but 
it is more future proof.

Paolo