linux-kernel - Re: [PATCH] x86/speculation: Add document to describe Spectre and its mitigations

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Message-ID: <0d89b5a0-01d3-4927-ce59-fd9dc24c53b3@linux.intel.com>
Date:   Mon, 7 Jan 2019 09:57:19 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Ben Greear <greearb@...delatech.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH] x86/speculation: Add document to describe Spectre and its
 mitigations

On 12/31/18 8:22 AM, Ben Greear wrote:
> 
> 
> On 12/21/2018 05:17 PM, Tim Chen wrote:
>>
>> If you don't worry about security and performance is paramount, then
>> boot with "nospectre_v2".  That's explained in the document.
> 
> There seem to be lots of different variants of this type of problem.  It was not clear
> to me that just doing nospectre_v2 would be sufficient to get back full performance.

The performance penalty comes from retpoline penalizing indirect branch predictions in kernel.
With nospectre_v2, retpoline is disabled so you should get all the performance
back from spectre mitigation.

This does not disable kernel page table isolation for meltdown mitigation, which also
needs to be turned off if you want to get the full performance back.  That's somewhat
beyond the scope of this doc on Spectre.

> 
> And anyway, I would like to compile the kernel to not need that command-line option,
> so I am still interesting in what compile options need to be set to what values...
> 

If you just want to disable spectre mitigation, setting CONFIG_RETPOLINE=n should do
the  trick.  If you also want to disable meltdown mitigation,
set CONFIG_PAGE_TABLE_ISOLATION=n.

Thanks.

Tim