linux-kernel - Re: [PATCH v3 09/10] x86/split_lock: Define #AC for split lock feature

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Date:   Mon, 4 Feb 2019 11:00:03 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Chan <michael.chan@...adcom.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v3 09/10] x86/split_lock: Define #AC for split lock
 feature

On 2/4/19 10:45 AM, Fenghua Yu wrote:
> On Mon, Feb 04, 2019 at 10:41:40AM -0800, Dave Hansen wrote:
>> On 2/1/19 9:14 PM, Fenghua Yu wrote:
>>> --- a/arch/x86/include/asm/cpufeatures.h
>>> +++ b/arch/x86/include/asm/cpufeatures.h
>>> @@ -221,6 +221,7 @@
>>>  #define X86_FEATURE_ZEN			( 7*32+28) /* "" CPU is AMD family 0x17 (Zen) */
>>>  #define X86_FEATURE_L1TF_PTEINV		( 7*32+29) /* "" L1TF workaround PTE inversion */
>>>  #define X86_FEATURE_IBRS_ENHANCED	( 7*32+30) /* Enhanced IBRS */
>>> +#define X86_FEATURE_AC_SPLIT_LOCK	( 7*32+31) /* #AC for split lock */
>>
>> The last time this was posted, we (Intel) promised to go get the proper
>> (CPUID or MSR-based) enumeration of this feature documented.  Did we do
>> that?  If so, where is that documentation?
> 
> As said in the cover patch:
> "Please note: The feature could be enumerated through
> MSR IA32_CORE_CAPABILITY (0xCF). But the enumeration is not completely
> published yet. So this patch set doesn't include the method."

FWIW, I think we (Intel) promised to get this documented _before_ we
came back asking our intrepid x86 maintainers to accept something like
setcpuid=.  While I generally applaud the post-early-post-often
attitude, we might have been a bit trigger-happy on this one.

This is awfully crucial information to bury in a massive cover letter,
especially when the patch description is a rather anemic two lines.