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 14:21:02 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Jim Mattson <jmattson@...gle.com>,
        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>,
        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, Jan 31, 2018 at 2:10 PM, David Woodhouse <dwmw2@...radead.org> wrote:
>
> 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*.

Amen, brother!

Please please please can Amazon and friends push this? The current
situation with IBRS_ALL is complete nasty horrible garbage. It's
pointless on current CPU's, and it's not well-defined enough on future
CPU's.

The whole "you can enable this, but performance may or may not be
acceptable, and we won't tell you" thing is some bad mumbo-jumbo.

Before IBRS is good, we'll do retpoline and BTB stuffing and have
those (hopefully very rare) IBPB's. So the whole "badly performing
IBRS_ALL" is completely pointless, and actively wrong.

           Linus