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

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Message-ID: <a8f25ab3-0b1d-a248-9564-238e3dca3da4@amazon.com>
Date:   Wed, 31 Jan 2018 23:52:27 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Jim Mattson <jmattson@...gle.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 01/31/2018 09:18 PM, Jim Mattson wrote:
> 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.

I actually got rid of this save_spec_ctrl_on_exit variable and replaced
it with another variable like the one suggested for IBPB. Just to avoid
doing an expensive guest_cpuid_has. Now I peak instead in the MSR bitmap
to figure out if this MSR was supposed to be intercepted or not. This
test should provide a similar semantics to save_spec_ctrl_on_exit.

Anyway, cleaning up/testing now and will post a new version.
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