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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 31 Jan 2018 11:53:52 -0800
Message-ID: <CALMp9eRgc-qKS5CDTvqoUDdnAXu8xqy9ZB-rew=P9NgE5AfXWA@mail.gmail.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL
To:     KarimAllah Ahmed <karahmed@amazon.de>
Cc:     kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>
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On Wed, Jan 31, 2018 at 11:37 AM, KarimAllah Ahmed <karahmed@amazon.de> wrote:

> +
> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
> +               nested_vmx_disable_intercept_for_msr(
> +                               msr_bitmap_l1, msr_bitmap_l0,
> +                               MSR_IA32_SPEC_CTRL,
> +                               MSR_TYPE_R | MSR_TYPE_W);
> +       }
> +

As this is written, L2 will never get direct access to this MSR until
after L1 writes it.  What if L1 never writes it? The condition should
really be something that captures, "if L0 is willing to yield this MSR
to the guest..."