linux-kernel - Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting

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Date:   Fri, 27 Jul 2018 15:58:13 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Kyle Huey <me@...ehuey.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Len Brown <len.brown@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        David Matlack <dmatlack@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andi Kleen <andi@...stfloor.org>,
        LKML <linux-kernel@...r.kernel.org>,
        user-mode-linux-devel@...ts.sourceforge.net,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-user@...ts.sourceforge.net>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting

On Fri, Jul 27, 2018 at 2:30 PM, Jim Mattson <jmattson@...gle.com> wrote:
> On Fri, Jul 27, 2018 at 2:05 PM, Andy Lutomirski <luto@...nel.org> wrote:
>> Does KVM *have* a concept of "maximum non-turbo frequency" of the
>> guest that it would make sense to expose here?  If so, presumably the
>> right solution is to expose it.
>
> KVM has the concept of a guest's invariant TSC frequency. The Maximum
> Non-Turbo Ratio is just some fraction of that. Sadly, the fraction is
> 100 MHz, 133.33MHz, or the "scalable bus frequency" from some other
> MSR, depending on microarchitecture.

Which is problematic, unless KVM wants to start deciding what the base
clock is.  There's MSR_FSB_FREQ, which is supported on Atom only,
IIRC.

I really wish Intel would get its act together.