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Patch 3 will use >> the existing CPU-hotplug callback interface: cpuhp_setup_state() with >> a down callback that returns -EBUSY to refuse the offline while SNP is >> active. That's the standard mechanism for conditionally preventing a >> CPU offline, and it keeps no private "hotplug disabled" state of its >> own — so there's nothing here for a getter on cpu_hotplug_disabled to >> replace. > > That's not a standard mechanism. That's a hack as you have to start the > offlining operation in order to prevent something you already know. > > The return code which prevents offlining is there for situations where > the subsystem/driver is momentarily in a state which cannot be > resolved. > > That's a very different story than knowing that state at the point of > installing the callback already. > Sure. >> I chose the cpuhp callback specifically over >> cpu_hotplug_disable_offlining(): the callback can be torn down with >> cpuhp_remove_state() when SNP is fully shut down, which re-enables CPU >> offlining. cpu_hotplug_disable_offlining() sets >> cpu_hotplug_offline_disabled, which is __ro_after_init and one-way — >> there's no interface to clear it, and adding one would mean dropping >> the __ro_after_init marking and a new core "re-enable offlining" >> API. So that route can't re-enable offlining on SNP shutdown without >> new core plumbing, whereas the cpuhp callback gives me that for free. > > That's exactly the wrong attitude. Hack around a core limitation in a > random driver by abusing the provided mechanism instead of sitting down > and doing the extra five lines of code which makes it entirely clear > what's going on. > > When Boris asked me how to disable hotplug, I had the impression that > this is about permanently preventing it. So I pointed him to > cpu_hotplug_disable_offlining(). > > If I had known that it's about temporary prevention during runtime of > something, then I'd pointed him to cpu_hotplug_disable()/enable() which > is five lines farther down in cpu.c. It's not rocket science to find > them. The first AI chatbot I asked pointed me to it immediately. > > cpu_hotplug_disable()/enable() is _the_ standard mechanism to prevent > hotplug operations temporarily. They return -EBUSY without invoking any > callback or changing any related state. > This is the interface i have been using, in fact this current patch (v8) is based on cpu_hotplug_disable()/enable(), but then this thread started from review feedback on the v8 patch using cpu_hotplug_disable()/enable() that it looks like a hack — the concern being that cpu_hotplug_disable() reads as a temporary lock rather than a way to enforce a basically-permanent system state. That's what led me to look at cpu_hotplug_disable_offlining() and a cpuhp down-callback as alternatives. Your point that cpu_hotplug_disable()/enable() is the standard mechanism to prevent hotplug operations temporarily settles it, and the disable/enable pair being reversible is exactly what's wanted here: it's undone when SNP is fully shut down, so it isn't actually permanent (unlike cpu_hotplug_disable_offlining(), which is one-way). So I'll stay with cpu_hotplug_disable()/enable() and drop the alternatives. Thanks, Ashish > So what's exactly the new core plumbing you need? > > Thanks, > > tglx >