linux-kernel - Re: [PATCH v3 4/4] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Date:   Tue, 30 Jan 2018 16:27:39 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v3 4/4] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

On Tue, Jan 30, 2018 at 4:19 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> The new code in nested_vmx_merge_msr_bitmap should be conditional on
> vmx->save_spec_ctrl_on_exit.

But then if L1 doesn't use MSR_IA32_SPEC_CTRL itself and it uses the
VM-entry MSR load list to set up L2's MSR_IA32_SPEC_CTRL, you will
never set vmx->save_spec_ctrl_on_exit, and L2's accesses to the MSR
will always be intercepted by L0.