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

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Message-ID: <1517259759.18619.38.camel@infradead.org>
Date:   Mon, 29 Jan 2018 21:02:39 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Arjan van de Ven <arjan@...ux.intel.com>,
        Eduardo Habkost <ehabkost@...hat.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        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@...r.kernel.org,
        x86@...nel.org, "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

On Mon, 2018-01-29 at 12:44 -0800, Arjan van de Ven wrote:
> On 1/29/2018 12:42 PM, Eduardo Habkost wrote:
> > 
> > The question is how the hypervisor could tell that to the guest.
> > If Intel doesn't give us a CPUID bit that can be used to tell
> > that retpolines are enough, maybe we should use a hypervisor
> > CPUID bit for that?
>
> the objective is to have retpoline be safe everywhere and never use IBRS
> (Linus was also pretty clear about that) so I'm confused by your question

The question is about all the additional RSB-frobbing and call depth
counting and other bits that don't really even exist for Skylake yet in
a coherent form.

If a guest doesn't have those, because it's running some future kernel
where they *are* implemented but not enabled because at *boot* time it
discovered it wasn't on Skylake, the question is what happens if that
guest is subsequently migrated to a Skylake-class machine.

To which the answer is obviously "oops, sucks to be you". So yes,
*maybe* we want a way to advertise "you might be migrated to Skylake"
if you're booted on a pre-SKL box in a migration pool where such is
possible. 

That question is a reasonable one, and the answer possibly the same,
regardless of whether the plan for Skylake is to use IBRS, or all the
hypothetical other extra stuff.
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