If QEMU can't discover cases where it won't work, what criteria should
the end user use to decide between the impls, or for that matter, what
criteria should a management api/app like libvirt use ? If the only decision
logic is 'try it & benchmark your VM' then its not a particularly useful
option.
I've basically got a choice of making libvirt always ad '-aio linux'
or never add it at all. My inclination is to the latter since it is
compatible with existing QEMU which has no -aio option. Presumably
'-aio linux' is intended to provide some performance benefit so it'd
be nice to use it. If we can't express some criteria under which it
should be turned on, I can't enable it; where as if you can express
some criteria, then QEMU should apply them automatically.
Pushing this choice of AIO impls to the app or user invoking QEMU just
does not seem like a win here.