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

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Date:   Mon, 29 Jan 2018 14:12:02 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        LKML <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>,
        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 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

On Mon, Jan 29, 2018 at 1:50 PM, Eduardo Habkost <ehabkost@...hat.com> wrote:
> On Mon, Jan 29, 2018 at 01:37:05PM -0800, Jim Mattson wrote:
>> For GCE, "you might be migrated to Skylake" is pretty much a
>> certainty. Even if you're in a zone that doesn't currently have
>> Skylake machines, chances are pretty good that it will have Skylake
>> machines some day in the not-too-distant future.
>
> This kind of scenario is why I suggest a "we promise you're not
> going to be migrated to Skylake" bit instead a "you may be
> migrated to Skylake" bit.  The hypervisor could prevent migration
> to Skylake hosts if management software chose to enable this bit,
> and guests would choose the safest option (i.e. assume the worst)
> if running on older hypervisors that don't set the bit.

Giving customers this option promises the logistical nightmare of
provisioning sufficient pre-Skylake-era machines in all pools until
sufficient post-Skylake-era machines can be deployed to replace them.

>> In general, making these kinds of decisions based on F/M/S is probably
>> unwise when running in a VM.
>
> Certainly.  That's why I suggest not trusting f/m/s unless the
> hypervisor is explicitly saying it's accurate.
>
> --
> Eduardo