linux-kernel - RE: [PATCH v6 2/5] KVM: x86: Add IBPB support

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Date:   Thu, 3 May 2018 12:46:02 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     kvm <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...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 v6 2/5] KVM: x86: Add IBPB support

> From: Paolo Bonzini
> Sent: Thursday, May 3, 2018 5:20 PM
> 
> On 03/05/2018 03:27, Wanpeng Li wrote:
> > So for 1) guest->guest attacks 2) guest/ring3->host/ring3 attacks 3)
> > guest/ring0->host/ring0 attacks, if IBPB is enough to protect these
> > three scenarios and retpoline is not needed?
> 
> In theory yes, in practice if you want to do that IBPB is much more
> expensive than retpolines, because you'd need an IBPB on vmexit or a
> cache flush on vmentry.
> 

yes if HT is disabled. otherwise IBPB alone is not sufficient since it's 
just one-time effect while poison from sibling thread can happen 
anytime. in latter case retpoline or IBRS is expected to use with
IBPB in conjunction as a full mitigation.

Thanks
Kevin