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

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Message-ID: <20201210130131.GP2414@hirez.programming.kicks-ass.net>
Date:   Thu, 10 Dec 2020 14:01:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        "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 Thu, Dec 10, 2020 at 01:22:02PM +0100, Paolo Bonzini wrote:
> On 10/12/20 13:14, Peter Zijlstra wrote:
> > On Thu, Dec 10, 2020 at 12:42:36PM +0100, Paolo Bonzini wrote:
> > > 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.
> > 
> > Core2 doesn't use TSC for timekeeping anyway. KVM shouldn't either.
> 
> On Core2, KVM guests pass TSC through kvmclock in order to get something
> usable and not incredibly slow.

Which is incredibly wrong.