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

From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Borislav Petkov <bp@alien8.de>, Fenghua Yu <fenghua.yu@intel.com>,
	Ingo Molnar <mingo@redhat.com>, H Peter Anvin <hpa@zytor.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Michael Chan <michael.chan@broadcom.com>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	Ricardo Neri <ricardo.neri@intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid
Date: Sun, 10 Feb 2019 20:20:20 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.21.1902102012520.8784@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190205170415.GG17550@hirez.programming.kicks-ass.net>

On Tue, 5 Feb 2019, Peter Zijlstra wrote:
> On Tue, Feb 05, 2019 at 07:19:16AM -0800, Dave Hansen wrote:
> > On 2/5/19 12:48 AM, Peter Zijlstra wrote:
> > This isn't something we want everybody and their grandma to turn on;
> > it's a rather specialized feature.  It's really only for folks that care
> > about the latency incurred across the entire system on split lock
> > operations.
> 
> That really should be everyone. That split lock stuff is horrible. There
> is no real down-side to having it always enabled. Code that breaks is
> bad code you want fixed anyway.
> 
> Like I said elsewhere, I wish it would #AC for any unaligned LOCK
> prefix, not just cross-line. I see why we'd not want to traditional RISC
> #AC for every load/store, but atomics really had better be aligned.

Right, we should really make this default enabled.

> > > Is this some transient state; where a few (early) models will not have
> > > the enumeration sorted but all later models will have it all neat and
> > > tidy?
> > 
> > From my understanding, it's not just an early stepping.  It's a
> > generational thing.  The current generation lacks the enumeration and
> > the next generation will get it.  Both have the silicon to implement the
> > feature itself.
> 
> I never said stepping, in fact I explicitly said model.
> 
> > > If so, we can easily do the FMS solution for this.
> > 
> > Yeah, we can.  I honestly forget why we didn't do FMS. :)
> 
> Right so FMS is fairly horrible; but when it is a stop-gap for a limited
> number of models it's waaay better than dodgy cmdline things.

One or two is fine. And _IF_ we get the enumeration sorted before we merge
that, then we can declare the FM list as immutable :)

> We could of course try to wrmsr_safe() detect the feature; but that
> might be a problem is the MSR exists on any other models and has a
> different meaning.

Well, yes, but that would be pretty stupid.

Thanks,

	tglx