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

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Message-ID: <0ec205ec-08e5-dba4-6eea-e3b3a8e3effe@amazon.com>
Date:   Wed, 31 Jan 2018 01:52:11 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.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>,
        "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 01/31/2018 01:27 AM, Jim Mattson wrote:
> 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.

I can add another variable (actually two) to indicate if msr
interception should be disabled or not for SPEC_CTRL and PRED_CMD in
nested case.

That would allow us to have a fast alternative to guest_cpuid_has in
nested_vmx_merge_msr_bitmap and at the same time maintain the current
semantics of save_spec_ctrl_on_exit (i.e we would still differentiate 
between set_msr that is called from the loading MSRs for the emulated 
vm-entry vs L2 actually writing to it).

What do you think?
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