linux-kernel - RE: [PATCH v4 2/5] KVM: x86: Add IBPB support

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Date:   Wed, 31 Jan 2018 18:04:36 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     KarimAllah Ahmed <karahmed@...zon.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Peter Zijlstra" <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: RE: [PATCH v4 2/5] KVM: x86: Add IBPB support


> On Wed, Jan 31, 2018 at 8:55 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> > In fact this MSR can even be passed down unconditionally, since it needs
> > no save/restore and has no ill performance effect on the sibling
> > hyperthread.
> 
> I'm a bit surprised to hear that IBPB has no ill performance impact on
> the sibling hyperthread. On current CPUs, this has to flush the BTB,
> doesn't it? And since the BTB is shared between hyperthreads, doesn't
> the sibling lose all of its branch predictions?

IBPB most definitely (in current implementations) will stop both hyperthreads for
the flush.

IBPB is not a cheap operation on a system level