linux-kernel - Re: [PATCH] x86/cpufeatures: Cleanup AMD speculation feature bits

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Date:   Fri, 26 Jan 2018 15:06:20 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     x86-ml <x86@...nel.org>, linux-tip-commits@...r.kernel.org,
        hpa@...or.com, gregkh@...uxfoundation.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org
Subject: Re: [PATCH] x86/cpufeatures: Cleanup AMD speculation feature bits

On 1/26/2018 12:49 PM, Borislav Petkov wrote:
> On Fri, Jan 26, 2018 at 06:45:18PM +0000, David Woodhouse wrote:
>> On Fri, 2018-01-26 at 19:41 +0100, Borislav Petkov wrote:
>>> +#define X86_FEATURE_AMD_IBPB           (13*32+12) /* "" Indirect Branch Prediction Barrier MSR */
>>
>> Stray quotes.
> 
> No no, those quotes are magical. :)
> 
> They don't show the string in /proc/cpuinfo, see arch/x86/kernel/cpu/mkcapflags.sh
> 
>>> +#define X86_FEATURE_IBRS               (13*32+14) /* Indirect Branch Restricted Speculation */
>>
>> Please don't call it that. Call it X86_FEATURE_AMD_IBRS and leave
>> X86_FEATURE_IBRS to parallel X86_FEATURE_IBPB as the *software* bit
>> which is vendor-independent.
> 
> Do we have a X86_FEATURE_IBRS bit?
> 
> If so, then this bit should become
> 
> #define X86_FEATURE_AMD_IBRS      (13*32+14) /* "" Speculation Control MSR only (AMD) */
> 
> *with* the magic quotes.

So I like the idea of AMD_IBRS/AMD_IBPB/AMD_STIBP and then use the magic
quotes as appropriate.  We could probably use the magic quotes on
AMD_STIBP and set X86_FEATURE_STIBP when we see X86_FEATURE_AMD_STIBP.

Just my two cents.

Thanks,
Tom

>