Re: [Xen-devel] /proc/xen/xenbus supports watch?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 20 Sep 2005, at 12:01, Rusty Russell wrote:

> By providing a kernel device node we
> make it look exactly like talking through a socket, so libxenstore is
> basically unchanged.  By each open using a separate page to talk to
> xenstored, we don't have to hold the kernel lock, nor worry about tool
> errors/crashes screwing the kernel's store page.  xenstored restarts 
> are
> transparent.  On migration, we can simply force close (unmap page,
> return EBADF), which libxestore can treat exactly like the xenstored
> restart case with sockets, reconnect and re-xmit.
None of this really adds weight for or against muxing versus 
page-per-transaction.
If accesses from domU userspace are always communicated via the kernel 
device node, userspace gets no direct access to the transport page and 
so the kernel can ensure the page does not get corrupted. Also there's 
no need to hold the kernel lock for the duration of the user-space 
transaction if we mux multiple transactions onto a single page -- as 
independent transactions it is up to xenstored to deal with sync issues 
between them. We would need to hold the lock for short periods during 
the transaction (e.g., while accessing the shared transport page) but 
we would not be holding it continuously.
As for client handling of migration/restart -- it's an API issue that 
is independent of the underlying 'wire' transport.
 -- Keir


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