Re: [RESEND PATCH v2 1/3] x86/fred: Allow variable-sized event frame

[Xin Li]

On 3/19/2025 6:55 AM, Brian Gerst wrote:
> > > > +/*
> > > > + * Initialize thread_info.user_pt_regs for IDT event delivery.
> > > > + *
> > > > + * For IDT user level event delivery, a pt_regs structure is pushed by both
> > > > + * hardware and software and always resides at a fixed offset from top of
> > > > + * current task kernel stack, thus thread_info.user_pt_regs is a per-task
> > > > + * constant and NEVER changes after initialization.
> > > > + *
> > > > + * While for FRED user level event delivery, user_pt_regs is updated in
> > > > + * fred_entry_from_user() immediately after user level event delivery.
> > > > + *
> > > > + * Note: thread_info.user_pt_regs of the init task is initialized at build
> > > > + * time.
> > > > + */
> > > > +void arch_init_user_pt_regs(struct task_struct *tsk)
> > > > +{
> > > > +       unsigned long top_of_stack = (unsigned long)task_stack_page(tsk) + THREAD_SIZE;
> > > > +
> > > > +       top_of_stack -= TOP_OF_KERNEL_STACK_PADDING;
> > > > +       tsk->thread_info.user_pt_regs = (struct pt_regs *)top_of_stack - 1;
> > > > +}
> > >
> > > Can this be put into arch_dup_task_struct() instead of creating another hook?
> >
> > I wanted to do it at the beginning but task stack is no longer part of
> > the task_struct on x86.  Make sense?
>
> I see that now.  My concern here is that using a weak function makes
> all other arches pay the cost of an empty function call.  That's why
> many hooks are static linlines or macros, especially if the default is
> a no-op.

Same here.

As you have gone through the code and logic, mind to give a RB?

Thanks!
    Xin