[PATCH v2] arm64: implement support for static call trampolines

[Ard Biesheuvel]

On Thu, 29 Oct 2020 at 12:46, Peter Zijlstra <peterz at infradead.org> wrote:
>
> On Thu, Oct 29, 2020 at 11:58:52AM +0100, Ard Biesheuvel wrote:
> > On Thu, 29 Oct 2020 at 11:40, Peter Zijlstra <peterz at infradead.org> wrote:
>
> > > Would it make things easier if your trampoline consisted of two complete
> > > slots, between which you can flip?
> > >
> > > Something like:
> > >
> > >         0x00    B 0x24 / NOP
> > >         0x04    < slot 1 >
> > >                 ....
> > >         0x20
> > >         0x24    < slot 2 >
> > >                 ....
> > >         0x40
> > >
> > > Then each (20 byte) slot can contain any of the variants above and you
> > > can write the unused slot without stop-machine. Then, when the unused
> > > slot is populated, flip the initial instruction (like a static-branch),
> > > issue synchronize_rcu_tasks() and flip to using the other slot for next
> > > time.
> > >
> >
> > Once we've populated a slot and activated it, we have to assume that
> > it is live and we can no longer modify it freely.
>
> Urhm how so? Once you pass through synchronize_rcu_tasks() nobody should
> still be using the old one.

But doesn't that require some RCU foo in the static call wrapper itself?