Messages in this thread | | | Date | Sun, 10 Feb 2019 20:20:20 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid |
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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
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