Messages in this thread | | | Date | Wed, 23 Nov 2022 09:03:37 -0800 | From | Josh Poimboeuf <> | Subject | Re: objtool warning for next-20221118 |
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On Wed, Nov 23, 2022 at 10:52:09AM +0000, Andrew Cooper wrote: > > Well, if you return from arch_cpu_idle_dead() you're back in the idle > > loop -- exactly where you would be if you were to bootstrap the whole > > CPU -- provided you have it remember the whole state (easier with a > > vCPU).
play_dead() really needs sane semantics. Not only does it introduce a surprise to the offlining code in do_idle(), it also skips the entire hotplug state machine. Not sure if that introduces any bugs, but at the very least it's subtle and surprising.
> > But maybe I'm missing something, lets add Xen folks on. > > Calling VCPUOP_down on oneself always succeeds, but all it does is > deschedule the vCPU. > > It can be undone at a later point by a different vcpu issuing VCPUOP_up > against the previously-downed CPU, at which point the vCPU gets rescheduled. > > This is why the VCPUOP_down hypercall returns normally. All state > really is intact. > > As for what Linux does, this is how xen_pv_cpu_up() currently behaves. > If you want to make Xen behave more everything else, then bug a BUG() > after VCPUOP_down, and adjust xen_pv_cpu_up() to skip its initialised > check and always use VCPUOP_initialise to bring the vCPU back online.
Or we could do what sev_es_play_dead() does and just call start_cpu0() after the hypercall returns?
Though, start_cpu0() seems very suspect, it just uses the stack of the idle task of whatever CPU happened to be last brought up via cpu_up(). Which means we now have two idle tasks sharing the same stack? How is start_cpu0() not broken???
-- Josh
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