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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 22 Sep 2011, Roger Pau Monn? 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.
> 
> 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, 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?

Give a look at hw/xen_backend.c and hw/xen_disk.c in upstream qemu,
they should give you an idea of what you need.
You would still need a way to access the grant table from userspace,
that on Linux is provided by the gntdev device, I hope there is
something similar in NetBSD.

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