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    SubjectRe: [RFC PATCH] x86: explicit call to mmiotrace in do_page_fault()
    >  	default n
    > help
    > This will build a kernel module called mmiotrace.
    > + Making this a built-in is heavily discouraged.

    why is this? Wouldn't it be nice if distros just shipped with this in their kernel by default
    for example?

    > -#ifdef CONFIG_PAGE_FAULT_HANDLERS
    > -static HLIST_HEAD(pf_handlers); /* protected by RCU */
    > -static DEFINE_SPINLOCK(pf_handlers_writer);
    > +#ifdef CONFIG_MMIOTRACE_HOOKS
    > +static pf_handler_func mmiotrace_pf_handler; /* protected by RCU */
    > +static DEFINE_SPINLOCK(mmiotrace_handler_lock);
    >
    > -void register_page_fault_handler(struct pf_handler *new_pfh)
    > +int mmiotrace_register_pf(pf_handler_func new_pfh)
    > {
    > + int ret = 0;
    > unsigned long flags;
    > - spin_lock_irqsave(&pf_handlers_writer, flags);
    > - hlist_add_head_rcu(&new_pfh->hlist, &pf_handlers);
    > - spin_unlock_irqrestore(&pf_handlers_writer, flags);
    > + spin_lock_irqsave(&mmiotrace_handler_lock, flags);
    > + if (mmiotrace_pf_handler)
    > + ret = -EBUSY;
    > + else
    > + mmiotrace_pf_handler = new_pfh;
    > + spin_unlock_irqrestore(&mmiotrace_handler_lock, flags);
    > + return ret;
    > }
    > -EXPORT_SYMBOL_GPL(register_page_fault_handler);
    > +EXPORT_SYMBOL_GPL(mmiotrace_register_pf);

    why not use a weak function instead? should be a ton simpler.


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