Re: [Qemu-devel] Question on ioeventfd behavior

[Paolo Bonzini]

On 26/02/2016 08:51, Markus Armbruster wrote:
> /**
>  * memory_region_add_eventfd: Request an eventfd to be triggered when a word
>  *                            is written to a location.
>  *
>  * Marks a word in an IO region (initialized with memory_region_init_io())
>  * as a trigger for an eventfd event.  The I/O callback will not be called.
>  * The caller must be prepared to handle failure (that is, take the required
>  * action if the callback _is_ called).
>  *
>  * @mr: the memory region being updated.
>  * @addr: the address within @mr that is to be monitored
>  * @size: the size of the access to trigger the eventfd
>  * @match_data: whether to match against @data, instead of just @addr
>  * @data: the data to match against the guest write
>  * @fd: the eventfd to be triggered when @addr, @size, and @data all match.
>  **/
> void memory_region_add_eventfd(MemoryRegion *mr,
>                                hwaddr addr,
>                                unsigned size,
>                                bool match_data,
>                                uint64_t data,
>                                EventNotifier *e);
> 
> What exactly does "The caller must be prepared to handle failure" mean?
> What are the failure modes?

I think it simply meant that the caller should be ready for the case
where ioeventfd is _not_ implemented (e.g. when running on TCG) and the
MMIO callback is invoked anyway.

memory.c now implements a generic fallback in
memory_region_dispatch_write_eventfds, so that should not be necessary
anymore.  I cannot think of other meanings of "failure mode".

In any case, ivshmem_io_write helpfully does the required
event_notifier_set, so you're good.

> Does "value written doesn't match" count as failure in the sense of this
> clause?  Rephrasing my question: what happens when the guest writes a
> value to the Doorbell register that does not match any of the values
> registered with memory_region_add_eventfd()?  Is the I/O callback
> called exactly as if ioeventfd was not in use?

Yes, the latter.

Paolo