linux-kernel - Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid

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Date:   Mon, 4 Feb 2019 15:24:23 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        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 08/10] x86/setcpuid: Add kernel option setcpuid

On 2/4/19 1:40 PM, Borislav Petkov wrote:
>> Then, for the weirdo deployments where this feature is not enumerated,
>> we have the setcpuid= to fake the enumeration in software.
>>
>> The reason I'm pushing for setcpuid= instead of a one-off is that I
>> don't expect this to be the last time Intel does this. I'd rather have
>> one setcpuid= than a hundred things like "ac_split_lock_disable".
> So my only issue with this is the user having to type this in in order
> to get the feature.
> 
> VS
> 
> automatically enabling it during boot in early_init_intel() or so. No
> need for any user intervention. It'll be just like a forgotten CPUID bit
> and we've done those before.

Actually, there's one part of all this that I forgot.  Will split lock
detection be enumerated _widely_?  IOW, will my laptop in 5 years
enumerate support for it?  If so, we surely don't want to enable this
everyhwhere: it will break old apps.  Doesn't that mean that we need
both feature detection and another separate bit for folks to opt-in?