Xen devel - Jun 2011 - How is front driver get probed and loaded?

Hi, all

I''ve finished a Xen prototype for virtio transport layer. I would like
to make sure it works as expected before I proceed to specific
drivers.

I''m planning to write simple test backend and dummy front driver. But
I have difficulty understanding how Xen front driver (e.g.
xen-blkfront) is probed and loaded.

Do I just need to setup corresponding xenstore entries and it will
work out of the box?

Thanks.

Wei.

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On Fri, 24 Jun 2011, Wei Liu wrote:
> Hi, all
> 
> I''ve finished a Xen prototype for virtio transport layer. I would
like
> to make sure it works as expected before I proceed to specific
> drivers.
> 
> I''m planning to write simple test backend and dummy front driver.
But
> I have difficulty understanding how Xen front driver (e.g.
> xen-blkfront) is probed and loaded.
> 
> Do I just need to setup corresponding xenstore entries and it will
> work out of the box?
Xen frontend drivers probing and configuration is all done through
xenstore. The toolstack writes the initial xenstore entries, then the
frontend and the backend fill up the rest.
There is a "state" node on xenstore that identifies in which state the
frontend and the backend are, where 4 is "connected".

I am not sure whether the existing xenstore protocol is going to help
you, I would probably try to stay closer to the behaviour of the virtio
pci protocol instead.

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On Fri, Jun 24, 2011 at 9:32 PM, Stefano Stabellini
<stefano.stabellini@eu.citrix.com> wrote:
> Xen frontend drivers probing and configuration is all done through
> xenstore. The toolstack writes the initial xenstore entries, then the
> frontend and the backend fill up the rest.
> There is a "state" node on xenstore that identifies in which
state the
> frontend and the backend are, where 4 is "connected".
>
I understand. What I''m missing is that the key to connect frontend and
backend, how do they recognize each other?

For example, if I need VBD support, backend will first write "vbd"
protocol entries in xenstore. Then domU boots. It reads xenstore and
finds "vbd" entries, then it will automatically load xen-blkfront. Is
this right?
> I am not sure whether the existing xenstore protocol is going to help
> you, I would probably try to stay closer to the behaviour of the virtio
> pci protocol instead.
>
By design this transport layer''s behavior resembles virtio pci
protocol. Things written to xenstore are only used to connect fe / be
transport layers. Lots of work is needed for QEMU backend.

Wei.

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On Fri, 24 Jun 2011, Wei Liu wrote:
> On Fri, Jun 24, 2011 at 9:32 PM, Stefano Stabellini
> <stefano.stabellini@eu.citrix.com> wrote:
> > Xen frontend drivers probing and configuration is all done through
> > xenstore. The toolstack writes the initial xenstore entries, then the
> > frontend and the backend fill up the rest.
> > There is a "state" node on xenstore that identifies in which
state the
> > frontend and the backend are, where 4 is "connected".
> >
> 
> I understand. What I''m missing is that the key to connect frontend
and
> backend, how do they recognize each other?
> 
> For example, if I need VBD support, backend will first write
"vbd"
> protocol entries in xenstore. Then domU boots. It reads xenstore and
> finds "vbd" entries, then it will automatically load
xen-blkfront. Is
> this right?
Close.
Libxenlight writes the "vbd" frontend and backend entries:

tools/libxl/libxl.c:libxl_device_disk_add

the xenbus drivers in the guest reads from xenstore and finds "vbd" so
it loads xen-blkfront; the same happens in dom0 with the backend.
After that, they establish a connection changing their state from 1 to
2, then from 2 to 3, until they reach 4.

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