Re: [Xen-devel] is a pure usermode backend possible?

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2010-11-19 at 09:59 +0000, James Harper wrote:
> Is there any reason why a purely user mode backend driver could not work
> in Dom0? I believe that the xenstore, grant table, and event channel
> interfaces all exist so I can't see why not,

Indeed, it's not only possible but already done, e.g. pvfb, file and the
console backends.

We are considering a userspace block backend right now. See the recent
thread "blktap: Sync with XCP, dropping zero-copy" and in particular
Stefano's experiments with the blkback implementation which is already
in upstream qemu.

>  unless creating nodes under
> /local/domain/0/backend might confuse something?

Only if you reused a subdirectory which was already used by a kernel
mode backend. There's no reason to do this though since the frontend
always finds the backend via its backend node in xenstore so you can
just pick a new name for your userspace backend.

The existing tap and vbd backends are an existing example of this
indirection, although both are in-kernel drivers in that case.

> Assuming it's possible, are there any performance reasons or other
> reasons why it might not be a good idea?

The general feeling is that a block backend completely in userspace may
well be acceptable (e.g. compared with tapdisk which already goes
through userspace) and Stefano's initial experiments seem to support
that.

We think that netback performance would suffer badly if you moved it to
userspace, although no one has done that experiment. AIUI the virtio
folks are currently looking at moving the virtio-net backend from user
to kernel for the same reason though.

Ian.


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