linux-kernel - Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

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Message-ID: <5f37b853-e7bb-0259-653c-6f526b106594@amazon.com>
Date:   Wed, 31 Jan 2018 18:11:39 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

On 01/31/2018 05:50 PM, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@...zon.de> wrote:
> 
>> +               vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
>> +                                             MSR_TYPE_W);
> 
> Why not disable this intercept eagerly, rather than lazily? Unlike
> MSR_IA32_SPEC_CTRL, there is no guest value to save/restore, so there
> is no cost to disabling the intercept if the guest cpuid info declares
> support for it.
> 
> 
>> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
>> +               nested_vmx_disable_intercept_for_msr(
>> +                               msr_bitmap_l1, msr_bitmap_l0,
>> +                               MSR_IA32_PRED_CMD,
>> +                               MSR_TYPE_R);
>> +       }
> 
> I don't think this should be predicated on
> "to_vmx(vcpu)->save_spec_ctrl_on_exit." Why not just
> "guest_cpuid_has(vcpu, X86_FEATURE_IBPB)"?

Paolo suggested this on the previous revision because guest_cpuid_has()
would be slow.

> Also, the final argument to
> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
> MSR_TYPE_R.
> 
Oops! will fix!
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