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

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Date:   Thu, 10 Dec 2020 12:26:18 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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

On Wed, Dec 09, 2020 at 09:58:23PM +0100, Thomas Gleixner wrote:
> 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? 

Upon VM resume, we notify NTP daemon in the guest to sync realtime
clock.
> 
> > 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