linux-kernel - Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Message-ID: <CALMp9eS9LAJz+u3ujKZ5kLHihHMZ68LHajn3ri7sRPPEgFbHfA@mail.gmail.com>
Date:   Wed, 31 Jan 2018 12:18:42 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     KarimAllah Ahmed <karahmed@...zon.com>
Cc:     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>,
        Paolo Bonzini <pbonzini@...hat.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 v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

On Wed, Jan 31, 2018 at 12:01 PM, KarimAllah Ahmed <karahmed@...zon.com> wrote:

> but save_spec_ctrl_on_exit is also set for L2 write. So once L2 writes
> to it, this condition will be true and then the bitmap will be updated.

So if L1 or any L2 writes to the MSR, then save_spec_ctrl_on_exit is
set to true, even if the MSR permission bitmap for a particular VMCS
*doesn't* allow the MSR to be written without an intercept. That's
functionally correct, but inefficient. It seems to me that
save_spec_ctrl_on_exit should indicate whether or not the *current*
MSR permission bitmap allows unintercepted writes to IA32_SPEC_CTRL.
To that end, perhaps save_spec_ctrl_on_exit rightfully belongs in the
loaded_vmcs structure, alongside the msr_bitmap pointer that it is
associated with. For vmcs02, nested_vmx_merge_msr_bitmap() should set
the vmcs02 save_spec_ctrl_on_exit based on (a) whether L0 is willing
to yield the MSR to L1, and (b) whether L1 is willing to yield the MSR
to L2.