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

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Date:   Fri, 27 Jul 2018 14:05:40 -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 2:03 PM, Jim Mattson <jmattson@...gle.com> wrote:
> On Fri, Jul 27, 2018 at 1:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Jul 27, 2018, at 1:28 PM, Jim Mattson <jmattson@...gle.com> wrote:
>>> Initializing this bit to zero helps with migration, but then if the
>>> CPUID faulting bits in both MSRs are set, userspace has to take pains
>>> to ensure that MSR_PLATFORM_INFO is restored first, or the
>>> MSR_MISC_FEATURES_ENABLES value will be rejected.
>>
>> The code could drop the constraint and just let the entry possibly fail if the MSRs are set wrong
>
> That would be an improvement, I think.

I personally don't know enough about the QEMU, kvmtool, etc
architecture to know whether this would be a good idea.

>
>>> I'm also concerned about the 0 in the "Maximum Non-Turbo Ratio" field
>>> feeding into someone's calculated TSC frequency.
>>
>> Hmm. I don’t have a good answer to that. Are there any real CPUs that have this MSR but don’t have that field?
>
> No. The reason I bring this up is that a customer has code that
> expects to be able to derive the TSC frequency from this MSR (per
> Intel's instructions in SDM volume 3, section 18.7.3), and they were
> surprised to find that the MSR raises #GP on our platform. We're
> looking into cherry-picking this support from upstream as a start, but
> I know the customer would be unhappy to read zero from bits 15:8.

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.