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

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Message-ID: <20180130204623.583b1a7a@alans-desktop>
Date:   Tue, 30 Jan 2018 20:46:23 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        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>,
        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 list <kvm@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

> If you are ever going to migrate to Skylake, I think you should just
> always tell the guests that you're running on Skylake. That way the
> guests will always assume the worst case situation wrt Specte.

Unfortunately if you do that then guest may also decide to use other
Skylake hardware features and pop its clogs when it finds out its actually
running on Westmere or SandyBridge.

So you need to be able to both lie to the OS and user space via cpuid and
also have a second 'but do skylake protections' that only mitigation
aware software knows about.

Alan