Re: [PATCH] x86/domain: restrict context switch hooks for the idle domain

[Roger Pau Monné <roger@xxxxxxxxxxxxxx>]

On Fri, Oct 02, 2026 at 07:45:07AM +0200, Jan Beulich wrote:
> On 01.10.2026 16:27, Roger Pau Monne wrote:
> > --- a/xen/arch/x86/domain.c
> > +++ b/xen/arch/x86/domain.c
> > @@ -813,8 +813,6 @@ static bool emulation_flags_ok(const struct domain *d, 
> > uint32_t emflags)
> >  void __init arch_init_idle_domain(struct domain *d)
> >  {
> >      static const struct arch_csw idle_csw = {
> > -        .from = paravirt_ctxt_switch_from,
> > -        .to   = paravirt_ctxt_switch_to,
> >          .tail = idle_loop,
> >      };
> 
> Leaving NULL pointers around isn't a good thing, though. May I suggest to
> at least poison the pointers then?

I don't mind doing so, but I don't think we usually do that for other
hook structures, ie: hvm_function_table for example doesn't poison
unset hooks (and it possibly can't, because there are .hook != NULL
checks).

TBH I'm missing a benefit of the poisoning here.  Using NULL or a
poisoned value will both trigger a page-fault, and NULL has the
benefit of callers easily checking whether the hook is set.

Thanks, Roger.