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

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Date:   Wed, 31 Jan 2018 22:10:24 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        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>,
        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, 2018-01-31 at 14:06 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 1:59 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> > I'm actually working on IBRS_ALL at the moment.
> >
> > I was tempted to *not* let the guests turn it off. Expose SPEC_CTRL but
> > just make it a no-op.
> 
> Maybe we could convince Intel to add a LOCK bit to IA32_SPEC_CTRL like
> the one in IA32_FEATURE_CONTROL.

Given that IBRS_ALL is supposed to be a sanely-performing option, I'd
rather convince Intel to just make it unconditional. If they've added
the appropriate tagging to the BTB, why even *have* this deliberately
insecure mode when IBRS==0?

I understand that until/unless they get a *proper* fix, software is
still going to have to use IBPB as appropriate. But there's no need for
the IBRS bit to do *anything*.
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