Re: [Xen-devel] Most up to date blktap2 kernel driver

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Thu, 2011-09-22 at 09:59 +0100, Roger Pau Monnà wrote:
> > I was just wondering about this, is your plan to bolt a block frontend
> > onto the existing tapdisk2 process or are you building a thing using a
> > NetBSD library for doing block devices in userspace (I think you are
> > doing the former but either you or Christoph mentioned that library
> > thing at one point too).
> 
> Well, my first try was to create a user-space implementation of the
> block devices found in blktap, so there was no need to change the
> current blktap tools. This is however not possible right now, because
> the library for implementing block and char devices as user-space
> daemons doesn't support mmap, and there's no clear way about how to
> implement this.

Right. And that would be very OS specific I guess?

> Correct me if I'm wrong, but the second option would be to directly
> implement the I/O ring from user-space without using any devices,

Yes, that is what I was suggesting. I think we generally have consensus
that this is the right direction to take blktap.

> I haven't had much time to look into this, but I think this is the best
> way, because it doesn't involve the constant context changes that the
> implementation explained before has. I hope that looking into qemu
> disk backend file can provide me with some information about how to do
> this, possibly using libxc?

Using libxc would be better than using the OS provided interfaces
directly since it abstracts out the differences between the
ioctls/devices on different OSs.

> 
> > The former plan would be awesome since it would make this work useful on
> > all platforms and not just NetBSD. As you've no doubt noticed the
> > blktap2 kernel side isn't (and won't be) in Linux upstream either.
> >
> > Ian.



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