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

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Date:   Tue, 5 Feb 2019 08:46:23 -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 10:18 PM, Borislav Petkov wrote:
> On Mon, Feb 04, 2019 at 03:24:23PM -0800, Dave Hansen wrote:
>> Actually, there's one part of all this that I forgot.  Will split lock
>> detection be enumerated _widely_?
> 
> You never know what users will do. The moment it gets out, it better be
> designed properly, along with the chicken bits.

Sure.  I think this was just the simplest implementation we could come
up with.  There was more complexity before, and Thomas suggested
stripping it back to the bare-bones like we have here.

>> IOW, will my laptop in 5 years enumerate support for it?
> 
> Don't tell me this is going to be another MPX fiasco :-\
> 
> Or is this something along the lines of "let's see whether it takes off
> and if yes, we'll commit to it or otherwise remove it and not even waste
> a CPUID leaf"?

"Is Intel serious enough to put in a CPUID leaf" is a pretty good litmus
test INMHO.  I think it's one of the reasons that Thomas said he would
consider this if Intel was willing to go to the trouble of adding proper
enumeration.

>> 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?
> 
> Well, if it breaks old apps, it probably needs to be opt-in anyway.

Yes, this was my assumption.

> And for that you don't need setcpuid either - you simply boot with
> "split_lock_ac" or whatever and the kernel pokes that MSR_TEST_CTL or
> whatever else it needs to detect in hw for split lock and sets the
> X86_FEATURE bits if detection is successful.

That's actually what we did in the last set.

Anyway...  There are a few branches of this discussion.  Let's wait for
Fenghua to tell us how universal this feature is and if
family/model/stepping detection will work.