linux-kernel - Re: [PATCH v5 2/5] KVM: x86: Add IBPB support

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Date:   Wed, 31 Jan 2018 19:53:18 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.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>
Subject: Re: [PATCH v5 2/5] KVM: x86: Add IBPB support



On Wed, 2018-01-31 at 11:45 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 11:37 AM, KarimAllah Ahmed  wrote:
> 
> > +       nested_vmx_disable_intercept_for_msr(msr_bitmap_l1, msr_bitmap_l0,
> > +                                            MSR_IA32_PRED_CMD,
> > +                                            MSR_TYPE_W);
> > +
> 
> I still think this should be predicated on L1 having
> guest_cpuid_has(vcpu, X86_FEATURE_IBPB) or guest_cpuid_has(vcpu,
> X86_FEATURE_SPEC_CTRL), because of the potential impact to the
> hypertwin. If L0 denies the feature to L1 by clearing those CPUID
> bits, L1 shouldn't be able to bypass that restriction by launching L2.

Rather than doing the expensive guest_cpu_has() every time (which is
worse now as we realised we need two of them) perhaps we should
introduce a local flag for that too?
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