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

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Message-ID: <06cb88da-f355-41ed-380f-7daa8ddf6159@amazon.com>
Date:   Wed, 31 Jan 2018 21:01:33 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     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 01/31/2018 08:53 PM, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 11:37 AM, KarimAllah Ahmed <karahmed@...zon.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..."

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.

> 
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