[Xen-devel] Re: How is front driver get probed and loaded?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 24 Jun 2011, Wei Liu wrote:
> On Fri, Jun 24, 2011 at 9:32 PM, Stefano Stabellini
> <stefano.stabellini@xxxxxxxxxxxxx> 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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