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

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Message-ID: <87r1nyzogg.fsf@nanos.tec.linutronix.de>
Date:   Wed, 09 Dec 2020 21:58:23 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, 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

Marcelo,

On Wed, Dec 09 2020 at 13:34, Marcelo Tosatti wrote:
> On Tue, Dec 08, 2020 at 10:33:15PM +0100, Thomas Gleixner wrote:
>> On Tue, Dec 08 2020 at 15:11, Marcelo Tosatti wrote:
>> > max_cycles overflow. Sent a message to Maxim describing it.
>> 
>> Truly helpful. Why the hell did you not talk to me when you ran into
>> that the first time?
>
> Because 
>
> 1) Users wanted CLOCK_BOOTTIME to stop counting while the VM 
> is paused (so we wanted to stop guest clock when VM is paused anyway).

How is that supposed to work w/o the guest kernels help if you have to
keep clock realtime up to date? 

> 2) The solution to inject NMIs to the guest seemed overly
> complicated.

Why do you need NMIs?

All you need is a way to communicate to the guest that it should prepare
for clock madness to happen. Whether that's an IPI or a bit in a
hyperpage which gets checked during the update of the guest timekeeping
does not matter at all.

But you certainly do not need an NMI because there is nothing useful you
can do within an NMI.

Thanks,

        tglx