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

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Date:   Wed, 31 Jan 2018 11:55:52 -0500
From:   Paolo Bonzini <pbonzini@...hat.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>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

On 31/01/2018 11:50, Jim Mattson wrote:
>> +       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)"? Also, the final argument to
> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
> MSR_TYPE_R.

In fact this MSR can even be passed down unconditionally, since it needs
no save/restore and has no ill performance effect on the sibling
hyperthread.

Only MSR_IA32_SPEC_CTRL needs to be conditional on
"to_vmx(vcpu)->save_spec_ctrl_on_exit".

Paolo