linux-kernel - Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

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Date:   Mon, 29 Jan 2018 14:41:24 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     David Dunn <ddunn@...are.com>
Cc:     Eduardo Habkost <ehabkost@...hat.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        "Wilson, Matt" <msw@...zon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Fred Jacobs <fjacobs@...are.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

> Even if we expose bit to indicate that FMS matches the underlying host, when does the guest know to query that?  The VM can be moved at any point in time, including after the guest asks if FMS matches host.

There's no way to enable these mitigations later, so if you always
have to enable the super set of all the mitigations for all the hosts you
might be migrating too.  

As of currently that means if you want to ever migrate to Skylake you should
set the Skylake model number and you're good.

-Andi