[Qemu-devel] [RFC 0/5] nbd: Adapt for dataplane

[Max Reitz]

For the NBD server to work with dataplane, it needs to correctly access
the exported BDS. It makes the most sense to run both in the same
AioContext, therefore this series implements methods for tracking a
BDS's AioContext and makes NBD make use of this for keeping the clients
connected to that BDS in the same AioContext.

The reason this is an RFC and not a PATCH is my inexperience with AIO,
coroutines and the like. Also, I'm not sure about what to do about the
coroutines. The NBD server has up to two coroutines per client: One for
receiving and one for sending. Theoretically, both have to be
"transferred" to the new AioContext if it is changed; however, as far as
I see it, coroutines are not really bound to an AioContext, they are
simply run in the AioContext entering them. Therefore, I think a
transfer is unnecessary. All coroutines are entered from nbd_read() and
nbd_restart_write(), both of which are AIO routines registered via
aio_set_fd_handler2().

As bs_aio_detach() unregisters all of these routines, the coroutines can
no longer be entered, but only after bs_aio_attach() is called again.
Then, when they are called, they will enter the coroutines in the new
AioContext. Therefore, I think an explicit transfer unnecessary.

However, if bs_aio_detach() is called from a different thread than the
old AioContext is running in, we may still have coroutines running for
which we should wait before returning from bs_aio_detach().

But because of my inexperience with coroutines, I'm not sure. I now have
these patches nearly unchanged here for about a week and I'm looking for
ways of testing them, but so far I could only test whether the old use
cases work, but not whether they will work for what they are intended to
do: With BDS changing their AioContext.

So, because I'm not sure what else to do and because I don't know how to
test multiple AIO threads (how do I move a BDS into another iothread?)
I'm just sending this out as an RFC.


Max Reitz (5):
  nbd: Correct name comparison for export_set_name()
  aio: Add io_read_poll() callback
  nbd: Use aio_set_fd_handler2()
  block: Add AIO followers
  nbd: Follow the BDS' AIO context

 aio-posix.c               | 26 ++++++++++++++++-----
 block.c                   | 55 +++++++++++++++++++++++++++++++++++++++++++
 include/block/aio.h       | 12 ++++++++++
 include/block/block_int.h | 40 ++++++++++++++++++++++++++++++++
 include/qemu/main-loop.h  |  1 -
 nbd.c                     | 59 ++++++++++++++++++++++++++++++++++++++---------
 6 files changed, 175 insertions(+), 18 deletions(-)

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1.9.3