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

[Andy Lutomirski]

On Tue, Dec 15, 2020 at 5:32 PM Ira Weiny <ira.weiny@xxxxxxxxx> wrote:
>
> On Fri, Dec 11, 2020 at 02:14:28PM -0800, Andy Lutomirski wrote:
> > On Mon, Nov 23, 2020 at 10:10 PM <ira.weiny@xxxxxxxxx> wrote:

> > 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.  And we have a little
> > accessor like:
> >
> > struct extended_pt_regs *extended_regs(struct pt_regs *) { return
> > container_of(...); }
> >
> > And we tell eBPF that extended_pt_regs is NOT ABI, and we will change
> > it whenever we feel like just to keep you on your toes, thank you very
> > much.
> >
> > Does this seem reasonable?
>
> Conceptually yes.  But I'm failing to see how this implementation can be made
> generic for the generic fields.  The pks fields, assuming they stay x86
> specific, would be reasonable to add in PUSH_AND_CLEAR_REGS.  But the
> rcu/lockdep field is generic.  Wouldn't we have to modify every architecture to
> add space for the rcu/lockdep bool?
>
> If not, where is a generic place that could be done?  Basically I'm missing how
> the effective stack structure can look like this:
>
> > struct extended_pt_regs {
> >   bool rcu_whatever;
> >   other generic fields here;
> >   struct arch_extended_pt_regs arch_regs;
> >   struct pt_regs regs;
> > };
>
> It seems more reasonable to make it look like:
>
> #ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
> struct extended_pt_regs {
>         unsigned long pkrs;
>         struct pt_regs regs;
> };
> #endif
>
> And leave the rcu/lockdep bool passed by value as before (still in C).

We could certainly do this, but we could also allocate some generic
space.  PUSH_AND_CLEAR_REGS would get an extra instruction like:

subq %rsp, $GENERIC_PTREGS_SIZE

or however this should be written.  That field would be defined in
asm-offsets.c.  And yes, all the generic-entry architectures would
need to get onboard.

If we wanted to be fancy, we could split the generic area into
initialize-to-zero and uninitialized for debugging purposes, but that
might be more complication than is worthwhile.