Re: [Qemu-devel] [RFC v1 08/22] memory: provide defaults for MemoryListener operations

[Anthony Liguori]

Avi Kivity <address@hidden> writes:

> On 10/04/2012 04:05 PM, Anthony Liguori wrote:
>> Avi Kivity <address@hidden> writes:
>> 
>>> Many listeners don't need to respond to all MemoryListener callbacks;
>>> provide suitable defaults instead.
>>>
>
>>> +#define MEMORY_LISTENER_DEFAULT_OPS                         \
>>> +    .begin = memory_listener_default_global,                \
>>> +    .commit = memory_listener_default_global,               \
>>> +    .region_add = memory_listener_default_section,          \
>>> +    .region_del = memory_listener_default_section,          \
>>> +    .region_nop = memory_listener_default_section,          \
>>> +    .log_start = memory_listener_default_section,           \
>>> +    .log_stop = memory_listener_default_section,            \
>>> +    .log_sync = memory_listener_default_section,            \
>>> +    .log_global_start = memory_listener_default_global,     \
>>> +    .log_global_stop = memory_listener_default_global,      \
>>> +    .eventfd_add = memory_listener_default_eventfd,         \
>>> +    .eventfd_del = memory_listener_default_eventfd          \
>>> +
>>> +void memory_listener_default_global(MemoryListener *listener);
>>> +void memory_listener_default_section(MemoryListener *listener,
>>> +                                     MemoryRegionSection *section);
>>> +void memory_listener_default_eventfd(MemoryListener *listener,
>>> +                                     MemoryRegionSection *section,
>>> +                                     bool match_data, uint64_t data, 
>>> EventNotifier *e);
>>> +
>>>  /**
>>>   * memory_region_init: Initialize a memory region
>>>   *
>> 
>> I think it'd be nicer to check for NULL when invoking the functions in
>> the memory core.
>> 
>> Then you avoid the exported stub functions entirely.
>
> Yes, that's the common style, but I happen not to like the extra check,
> both from a performance point of view (doesn't apply here of course) and
> from a readability point of view.

The trouble with your approach is that it introduced a subtle behavior
based on ordering.   IOW:

MemoryListenerOps foo = {
    MEMORY_LISTENER_DEFAULT_OPS,
    .log_sync = ...,
};

vs.

MemoryListenerOps foo = {
    .log_sync = ...,
    MEMORY_LISTENER_DEFAULT_OPS,
};

Both compile fine but have potentially difficult to debug differences.
Relying on zero-initialization eliminates the possibility of this problem.

Regards,

Anthony Liguori

>
> -- 
> error compiling committee.c: too many arguments to function