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

[Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>]

On Wed, Sep 14, 2005 at 10:21:04AM +1000, Rusty Russell wrote:
> I think if we want to do this we should actually introduce a new
> mechanism for communications.  There's no reason why a domain can't
> introduce more pages for xenstore communication beyond the one it is
> given to start with, is there?  (OK, unintroduce needs to take a shared
> page instead/as well as a domid).
> 
> So when someone opens the xenbus dev, we introduce a new page to the
> domain and the xenstored uses that for comms.  When closed, the page is
> released.  This actually simplifies the xenbus_dev driver a lot: now
> it's just a dumb pass-through since we don't have to worry about the
> userspace program blowing chunks all over the kernel's comms mechanism.

That would work but there's a few reasons why this is impracticle:
- xenstored needs to keep track of these pages and this consumes
  resources in the server.  There's no way for xenstored to close
  a connection to free up resources unless we want to start handling
  this case in all clients.  With multiple transactions, we can just
  timeout the transaction after 5 minutes, we have to handle the
  timeout case in the client already anyway.
- this complicates save/restore yet again because we'll have to
  reconnect all these pages when the domain is restored

    christian


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