Re: Folllowing up on LSF/MM RCU/idle discussion

[Peter Zijlstra]

On Mon, May 09, 2022 at 08:56:33AM -0700, Paul E. McKenney wrote:
> Hello, Jiri!
> 
> It was good chatting with you last week, and I hope that travels went
> well!
> 
> Just wanted to follow up on the non-noinstr code between the call
> to rcu_idle_enter() and rcu_idle_exit().  Although the most correct
> approach is to never have non-noinstr code in arch_cpu_idle(), for all I
> know there might well be architectures for which this is not feasible.
> If so, one workaround would be to supply a flag set by each arch (or
> subarch) that says that rcu_idle_enter() and rcu_idle_exit() are invoked
> within arch_cpu_idle().
> 
> CCing Peter, who just might have an opinion on this.  ;-)

Definitely have an opinion; just lack the tools to enforce these rules.
I cleaned up the worst of it for x86 but it's a shit-show for most
others. ARM in particular has some 'issues'.

But yeah, noinstr only when you do rcu_idle_enter.

The problem with validating all this is that cpuidle is a rats nest of
indirect calls; in order to validate the noinstr'ness of something like
that we need compiler support for pointer address spaces such that we
can stick pointers to noinstr functions in a different address space and
get complaints etc..