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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?