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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 31 Jan 2018 08:50:40 -0800
Message-ID: <CALMp9eRKz+sKeah0V0bpiYJJUgTM5oisbtahkNGtW=seW2W5JQ@mail.gmail.com>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support
To:     KarimAllah Ahmed <karahmed@amazon.de>
Cc:     "the arch/x86 maintainers" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        David Woodhouse <dwmw@amazon.co.uk>
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On Wed, Jan 31, 2018 at 5:10 AM, KarimAllah Ahmed <karahmed@amazon.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)"? Also, the final argument to
nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
MSR_TYPE_R.