Re: [KVM PATCH v4 3/3] kvm: add iosignalfd support

[Gregory Haskins]

Avi Kivity wrote:
> Gregory Haskins wrote:
>>
>>> What happens if you register to iosignalfds for the same address but
>>> with different cookies (a very practical scenario)?
>>>     
>>
>> This is really only supported at the iosignal interface level.  Today,
>> you can do this and the registration will succeed, but at run-time an
>> IO-exit will stop at the first in_range() hit it finds.  Therefore, you
>> will only get service on the first/lowest registered range.
>>
>> I knew this was a limitation of the current io_bus, but I put the
>> feature into iosignalfd anyway so that the user/kern interface was
>> robust enough to support the notion should we ever need it (and can thus
>> patch io_bus at that time).  Perhaps that is short-sighted because
>> userspace would never know its ranges weren't really registered
>> properly.
>>
>> I guess its simple enough to have io_bus check all devices for a match
>> instead of stopping on the first.  Should I just make a patch to fix
>> this, or should I fix iosignalfd to check for in_range matches and fail
>> if it finds overlap?  (We could then add a CAP_OVERLAP_IO bit in the
>> future if we finally fix the io_bus capability).  I am inclined to lean
>> towards option 2, since its not known whether this will ever be useful,
>> and io_bus scanning is in a hot-path.
>>
>> Thinking about it some more, I wonder if we should just get rid of the
>> notion of overlap to begin with.  Its a slippery slope (should we also
>> return to userspace after scanning and matching io_bus to see if it has
>> any overlap too?).  I am not sure if it would ever be used (real
>> hardware doesn't have multiple devices at the same address), and we can
>> always have multiple end-points mux from one iosignalfd if we really
>> need that.  Thoughts?
>>   
>
> Multiple cookies on the same address are required by virtio.  You
> can't mux since the data doesn't go anywhere.

Hmm..well, I might not be understanding properly, but I think we are
still ok.  IIUC, the concept is that we can register multiple
iosignalfds to trigger when a single range of [MM|P]IO is touched.  I.e.
one iowrite() triggers multiple eventfd_signal()s to go out.  You could
do this directly by having io_bus support multiple matches for
in_range().  You could also use a mux concept where one registration
fans out to multiple iosignalfds (either like you suggest below, or by
having one iosignalfd mux/relay to the others...I like your idea below
better, btw).

Or am I missing something?

>
>
> Virtio can survive by checking all rings on a notify, and we can later
> add a mechanism that has a distinct address for each ring, but let's
> see if we can cope with multiple cookies.  Mark?

I am confused by this.  I can totally see the use case for one
iosignalfd (w/ one address) for all rings (in a device), and one
iosignalfd per ring (each with a unique address).  But when would we
want to have one address serve multiple rings each with their own
notification?  Just curious.

>
>
> You could search existing iosignalfds for the same address and re-use
> the same iodevice.  I don't want to search the entire list since that
> precludes tricks like using hashtables or sorting the list by
> frequency of access.
>
Yeah, I like this idea best.  I can basically have my own "in_range"
mechanism inside the _iosignalfd structure.  I only register one range
with io_bus, but then I may have multiple targets within that.  I will
do this for v5.

-Greg

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