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

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Message-ID: <878sa82dnm.fsf@nanos.tec.linutronix.de>
Date:   Tue, 08 Dec 2020 22:25:49 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>,
        Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        kvm list <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 Tue, Dec 08 2020 at 09:33, Andy Lutomirski wrote:
> On Tue, Dec 8, 2020 at 8:26 AM Maxim Levitsky <mlevitsk@...hat.com> wrote:
>
> IIRC we introduced mul_u64_u32_shift() for roughly this reason,but we
> don't seem to be using it in the relevant code paths.  We should be
> able to use the same basic math with wider intermediates to allow very
> large intervals between updates.

That's 128 bit math and _NO_ we are not going there for various
reasons.

Hint1: There is a world outside of x8664 and TSC.
Hint2: Even on x8664 and TSC it comes with performance overhead

Thanks,

        tglx