On Tue, Feb 05, 2019 at 07:19:16AM -0800, Dave Hansen wrote: > On 2/5/19 12:48 AM, Peter Zijlstra wrote: > > On Mon, Feb 04, 2019 at 12:46:30PM -0800, Dave Hansen wrote: > >> So, the compromise we reached in this case is that Intel will fully > >> document the future silicon architecture, and then write the kernel > >> implementation to _that_. Then, for the weirdo deployments where this > >> feature is not enumerated, we have the setcpuid= to fake the enumeration > >> in software. > > > > What user is _EVER_ going to use this? Nobody, I expect the answer to > > be. > > This is one of the few times that we're pretty confident that folks will > use this. The reason we're going to this trouble is that the split lock > detection is wanted by actual customers, and they want it before it's > implemented on a processor with real enumeration.
That's big customers that do magic stuff not users. > 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. > > 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. 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. > > But a cmdline features thing is not something I can see anybody but > > a limited set of developers ever using. > > It's not for developers. This really is for (somewhat niche) end users > that want split lock detection in production. This is all really an > effort to get them running mainline or real distro kernels. Yes, because cloud service providers and RT are niche products.. *sigh*. Nobody wants to do live audio on their laptops.. oh wait..