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Re: [Xen-devel] reasons/requirements for some of patches/linux-*


On Fri, 2006-10-06 at 08:25 +0100, Jan Beulich wrote:
> Could anyone give a description (and reason it is needed for Xen, if that's
> not obvious from the description) for each of
> blktap-aio-16_03_06.patch

The blktap userland server (tapdisk) submits IO requests via kernel AIO,
but also waits for network/evtchn events via file descriptors.  And
alas, the kernel has no way to wait for AIO and fd wakeups in a single

This patch implements aio-for-poll --- it's a new core kernel ABI which
allows a temporary fd to be associated with an AIO IO context, so you
can wait on AIO and normal fds via a single select() call.  But this ABI
has been vetoed upstream.

Upstream has given initial approval to an alternative means whereby
instead of submitting a poll() for AIO, you submit an AIO request for
poll.  Then, io_getevents() can wait for both fd and AIO activity.  But
that functionality is still not upstream.

We did not want to include the non-approved AIO+poll mechanism in our
own Fedora kernels, so as a workaround, I've coded up a simple patch
which makes tapdisk spawn a separate thread for AIO events, and return
wakeups via a pipe fd to the main event loop when AIO arrives.  That
works fine as a temporary measure to allow us to ship blktap without the
AIO/poll stuff resolved upstream; but longer term, Jeff Moyer has been
talking to the AIO people to get epoll-for-AIO merged.  I'm not sure of
the status of that work though, it may well have stalled.

I can post the AIO-thread patch here if anyone's interested; I didn't
post it as part of my recent blktap changes because (a) Xen upstream
doesn't need it, it carries the aio-for-poll patch instead; and (b) the
_right_ answer for Xen is eventually to use an upstream-blessed poll-
for-aio patch instead.


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