linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Fri, 26 Jan 2018 08:46:57 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Liran Alon <liran.alon@...cle.com>, dave.hansen@...el.com
Cc:     labbott@...hat.com, luto@...nel.org, Janakarajan.Natarajan@....com,
        bp@...e.de, torvalds@...ux-foundation.org,
        asit.k.mallick@...el.com, rkrcmar@...hat.com, karahmed@...zon.de,
        hpa@...or.com, jun.nakajima@...el.com, mingo@...hat.com,
        x86@...nel.org, ashok.raj@...el.com, arjan.van.de.ven@...el.com,
        tim.c.chen@...ux.intel.com, pbonzini@...hat.com,
        ak@...ux.intel.com, linux-kernel@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de,
        gregkh@...uxfoundation.org, mhiramat@...nel.org,
        arjan@...ux.intel.com, thomas.lendacky@....com,
        dan.j.williams@...el.com, joro@...tes.org, aarcange@...hat.com,
        kvm@...r.kernel.org
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On Thu, 2018-01-25 at 18:11 -0800, Liran Alon wrote:
> 
> P.S:
> It seems to me that all these issues could be resolved completely at
> hardware in future CPUs if BTB/BHB/RSB entries were tagged with
> prediction-mode (or similar metadata). It will be nice if Intel/AMD
> could share if that is the planned long-term solution instead of
> IBRS-all-the-time.

IBRS-all-the-time is tagging with the ring and VMX root/non-root mode,
it seems. That much they could slip into the upcoming generation of
CPUs. And it's supposed to be fast¹; none of the dirty hacks in
microcode that they needed to implement the first-generation IBRS.

But we still need to tag with ASID/VMID and do proper flushing for
those, before we can completely ditch the need to do IBPB at the right
times.

Reading between the lines, I don't think they could add *that* without
stopping the fabs for a year or so while they go back to the drawing
board. But yes, I sincerely hope they *are* planning to do it, and
expose a 'SPECTRE_NO' bit in IA32_ARCH_CAPABILITIES, as soon as is
humanly possible.

¹ Fast enough that we'll want to use it and ALTERNATIVE out the 
  retpolines.
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