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DragonFly kernel List (threaded) for 2010-12
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Re: RFC: virtio blk driver

From: Alex Hornung <ahornung@xxxxxxxxx>
Date: Wed, 22 Dec 2010 08:07:39 +0000

> Pratyush and I have been working on the virtio drivers. The virtio-blk
> driver is ready for more testing and eyes looking at the code. The
> virtio-net driver is still in progress.

My first question: What is blkv2? How is it different from blk?

Regarding the code, this is definitely not in a good shape as far as
style(9) goes. The indentations are pretty random and there are a lot of
C++ comments to comment out code (//foo). Code should be commented out
with #if 0 ... #endif. It makes it pretty much a pain to read.

Now regarding the performance, is that the performance you were
expecting? How is the performance on NetBSD on the same system? If not
the same, where's the problem?

I'm also not so sure about the way you dispatch I/O in
virtio_disk_strategy. You essentially insert the I/O to be dispatched at
the end of a queue, then you call virtio_blk_execute, from exactly that
same point, and this then dispatches the first I/O on the queue. This
sounds quite dodgy and probably shouldn't be the case. How do you
guarantee all the enqueued I/Os are dispatched eventually? You can't
just assume you'll get more strategy calls. Maybe you are covering that
case, but from a short glance I don't see it. In any case, dispatching a
completely different I/O from the requested one in a strategy call might
not be the greatest idea for a plain block driver.


> The wiki has some notes about it: installing, architecture, porting
> notes, etc:

Regarding your 'Misc Porting Notes' on the wiki: Using spin locks to
protect the lists should be fine, as long as that's pretty much all they
are used for. spinlocks can't be used in blocking conditions; if that's
your intention, you'll have to use mutexes or lockmgr locks.

It seems your intentions are to make the design lockless? Do you have
any more detailed plans on this? One queue/ring per CPU, or what is your

Alex Hornung

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