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

Avi Kivity
2009-05-27T12:11:20Z

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.

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?

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.

--
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