linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Date:   Fri, 5 Jan 2018 14:01:00 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        David Woodhouse <dwmw@...zon.co.uk>
Cc:     ak@...ux.intel.com, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On 05/01/18 13:54, Thomas Gleixner wrote:
> On Thu, 4 Jan 2018, David Woodhouse wrote:
>> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
>> index 07cdd1715705..900fa7016d3f 100644
>> --- a/arch/x86/include/asm/cpufeatures.h
>> +++ b/arch/x86/include/asm/cpufeatures.h
>> @@ -342,5 +342,6 @@
>>  #define X86_BUG_MONITOR			X86_BUG(12) /* IPI required to wake up remote CPU */
>>  #define X86_BUG_AMD_E400		X86_BUG(13) /* CPU is among the affected by Erratum 400 */
>>  #define X86_BUG_CPU_INSECURE		X86_BUG(14) /* CPU is insecure and needs kernel page table isolation */
>> +#define X86_BUG_NO_RETPOLINE		X86_BUG(15) /* Placeholder: disable retpoline branch thunks */
> 
> I think this is the wrong approach. We have X86_BUG_CPU_INSECURE, which now
> should be renamed to X86_BUG_CPU_MELTDOWN_V3 or something like that. It
> tells the kernel, that the CPU is affected by variant 3.

MELTDOWN is variant 3.

> 
> If the kernel detects that and has PTI support then it sets the 'pti'
> feature bit which tells that the mitigation is in place.
> 
> So what we really want is
> 
>    X86_BUG_MELTDOWN_V1/2/3

X86_BUG_MELTDOWN, X86_BUG_SPECTRE_V1, X86_BUG_SPECTRE_V2


Juergen