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

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Message-Id: <6F48D384-B29C-41B4-83F1-B02FC2480205@amacapital.net>
Date:   Fri, 27 Jul 2018 13:46:06 -0700
From:   Andy Lutomirski <luto@...capital.net>
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 Jul 27, 2018, at 1:28 PM, Jim Mattson <jmattson@...gle.com> wrote:
> 
>> On Fri, Jul 27, 2018 at 12:41 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> On Wed, Feb 8, 2017 at 12:09 AM, Kyle Huey <me@...ehuey.com> wrote:
>>> Hardware support for faulting on the cpuid instruction is not required to
>>> emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant
>>> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
>>> cpuid-induced VM exit checks the cpuid faulting state and the CPL.
>>> kvm_require_cpl is even kind enough to inject the GP fault for us.
>>> 
>>> Signed-off-by: Kyle Huey <khuey@...ehuey.com>
>>> Reviewed-by: David Matlack <dmatlack@...gle.com>
>>> ---
>>> ...
>>> @@ -7613,16 +7636,19 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>> 
>>>        kvm_clear_async_pf_completion_queue(vcpu);
>>>        kvm_async_pf_hash_reset(vcpu);
>>>        vcpu->arch.apf.halted = false;
>>> 
>>>        if (!init_event) {
>>>                kvm_pmu_reset(vcpu);
>>>                vcpu->arch.smbase = 0x30000;
>>> +
>>> +               vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
>>> +               vcpu->arch.msr_misc_features_enables = 0;
>> 
>> Jim, I assume you're worried about this bit?  It seems like
>> msr_platform_info should maybe be initialized to zero to avoid causing
>> an unintended migration issue.
> 
> 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

> 
> 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?