Re: [PATCH V3.1] entry: Pass irqentry_state_t by reference

Peter Zijlstra
2020-12-17T13:19:49Z

On Thu, Dec 17, 2020 at 02:07:01PM +0100, Thomas Gleixner wrote:
> On Fri, Dec 11 2020 at 14:14, Andy Lutomirski wrote:
> > On Mon, Nov 23, 2020 at 10:10 PM <ira.weiny@intel.com> wrote:
> > After contemplating this for a bit, I think this isn't really the
> > right approach. It *works*, but we've mostly just created a bit of an
> > unfortunate situation. Our stack, on a (possibly nested) entry looks
> > like:
> >
> > previous frame (or empty if we came from usermode)
> > ---
> > SS
> > RSP
> > FLAGS
> > CS
> > RIP
> > rest of pt_regs
> >
> > C frame
> >
> > irqentry_state_t (maybe -- the compiler is within its rights to play
> > almost arbitrary games here)
> >
> > more C stuff
> >
> > So what we've accomplished is having two distinct arch register
> > regions, one called pt_regs and the other stuck in irqentry_state_t.
> > This is annoying because it means that, if we want to access this
> > thing without passing a pointer around or access it at all from outer
> > frames, we need to do something terrible with the unwinder, and we
> > don't want to go there.
> >
> > So I propose a somewhat different solution: lay out the stack like this.
> >
> > SS
> > RSP
> > FLAGS
> > CS
> > RIP
> > rest of pt_regs
> > PKS
> > ^^^^^^^^ extended_pt_regs points here
> >
> > C frame
> > more C stuff
> > ...
> >
> > IOW we have:
> >
> > struct extended_pt_regs {
> > bool rcu_whatever;
> > other generic fields here;
> > struct arch_extended_pt_regs arch_regs;
> > struct pt_regs regs;
> > };
> >
> > and arch_extended_pt_regs has unsigned long pks;
> >
> > and instead of passing a pointer to irqentry_state_t to the generic
> > entry/exit code, we just pass a pt_regs pointer.
>
> While I agree vs. PKS which is architecture specific state and needed in
> other places e.g. #PF, I'm not convinced that sticking the existing
> state into the same area buys us anything more than an indirect access.
>
> Peter?

Agreed; that immediately solves the confusion Ira had as well. While
extending pt_regs sounds scary, I think we've isolated our pt_regs
implementation from actual ABI pretty well, but of course, that would
need an audit. We don't want to leak this into signals for example.