Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <1517436624.18619.224.camel@infradead.org>
References: <1517427467-28567-1-git-send-email-karahmed@amazon.de>
 <1517427467-28567-5-git-send-email-karahmed@amazon.de> <CALMp9eRgc-qKS5CDTvqoUDdnAXu8xqy9ZB-rew=P9NgE5AfXWA@mail.gmail.com>
 <06cb88da-f355-41ed-380f-7daa8ddf6159@amazon.com> <CALMp9eS9LAJz+u3ujKZ5kLHihHMZ68LHajn3ri7sRPPEgFbHfA@mail.gmail.com>
 <67dca2bb-1298-c7fa-f681-018e4edc6725@redhat.com> <CALMp9eT677DU=EGoezcyjRMAP3XKiK_6FBJb-rVQrBX2CSbTuw@mail.gmail.com>
 <16d89e09-fbfc-4a08-00a6-40cfb4776018@redhat.com> <CALMp9eQ5fbfnqCvs_dMhoTOGTDX-+yt6QtJdUfiQg1BXq-0P3A@mail.gmail.com>
 <f6e76523-7727-6e79-7bcf-3d6c6ebd64d8@redhat.com> <CALMp9eRF4_khB40b1_HmUj9K+FPE5X0M+KUkthi7k_rdQdgwoA@mail.gmail.com>
 <1517435981.18619.215.camel@infradead.org> <CALMp9eQJieqaNSei-rQ4HwPq4jsZawB+R106ReJ__DR+XfuhNw@mail.gmail.com>
 <1517436624.18619.224.camel@infradead.org>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 31 Jan 2018 14:21:02 -0800
Message-ID: <CA+55aFw3-4dJpTG275CkVhBVUDHt6erDTBJeX+ei+rgzV6rJGQ@mail.gmail.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Jim Mattson <jmattson@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        KarimAllah Ahmed <karahmed@amazon.com>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jan 31, 2018 at 2:10 PM, David Woodhouse <dwmw2@infradead.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