linux-kernel - Re: [PATCH 1/7] x86/feature: Detect the x86 feature to control Speculation

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Date:   Fri, 5 Jan 2018 09:07:02 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] x86/feature: Detect the x86 feature to control
 Speculation

On 01/05/2018 07:14 AM, Tom Lendacky wrote:
> On 1/5/2018 5:14 AM, David Woodhouse wrote:
>> On Thu, 2018-01-04 at 09:56 -0800, Tim Chen wrote:
>>> cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature
>>> IA32_SPEC_CTRL (0x48) and IA32_PRED_CMD (0x49)
>>> IA32_SPEC_CTRL, bit0 – Indirect Branch Restricted Speculation (IBRS)
>>> IA32_PRED_CMD,  bit0 – Indirect Branch Prediction Barrier (IBPB)
>>
>> In a previous iteration of these patches, hadn't you already merged
>> support for the AMD variant? Where'd that go?
> 
> It looks like this series is strictly IBRS related.  Even though some of
> the IBPB definitions are in here, there isn't support later for indicating
> IBPB is in use or any places where IBPB would be used. I was assuming that
> there would be another series for IBPB support.  Is that the plan?

IBPB is meant to be a separate series.  We want to get retpoline
and IBRS out first.

> 
> AMD is covered by the IBRS support as is, but we also have support for
> the IBPB feature alone, identified by a different CPUID bit.  I was
> waiting for the IBPB series to see if I needed to submit anything or
> whether the patches were included.
> 

Tim