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Re: [Qemu-devel] Question on ioeventfd behavior


From: Markus Armbruster
Subject: Re: [Qemu-devel] Question on ioeventfd behavior
Date: Fri, 26 Feb 2016 10:58:53 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Paolo Bonzini <address@hidden> writes:

> 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.

Double-checking: can i rely on ivshmem_io_write() getting called on a
write of a non-matching value?



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