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List:       xen-devel
Subject:    Re: [Xen-devel] watches not working from domU userspace
From:       Keir Fraser <Keir.Fraser () cl ! cam ! ac ! uk>
Date:       2005-12-31 9:52:49
Message-ID: 15deca5b3628286d96c2d1e9441da756 () cl ! cam ! ac ! uk
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On 31 Dec 2005, at 05:38, Cihula, Joseph wrote:

> However, when the watch bookkeeping is combined with the transaction
> bookkeeping (which is already there) this is a lot of duplication with
> what xenstored already has to do for each connection.  I'm wondering if
> it wouldn't be better to add a sub-connection id to xenstored and 
> xenbus
> so that each xenbus client would be seen by xenstored as a separate
> (sub)connection.  Then watches and transactions would not need any
> special code in xenbus, as xenstored would handle the tracking and
> cleanup.
>
> This method would either need to change the xenstore wire protocol or
> co-opt the req_id field (which doesn't seem to be used).  While some
> additional code would have to be added to xenstored, I think that a 
> fair
> amount of code (and many synchronization locks/mutexes) in xenbus could
> be removed.

How about add a new command XS_NEW_CONNECTION, taking a grant reference 
and an event channel? This would then set up a new inter-domain event 
channel and messaging memory page, and then commands would no longer go 
via the kernel at all.

One drawback is it would then be very hard for the kernel to tell 
whether any xenstore transactions are in progress in user space. This 
is a problem since transaction state is not copied during save/restore. 
Either that needs to happen, or transactional xenstore code needs to 
handle 'spurious' failures at any point in a transaction (handle EAGAIN 
on any transactional read/write, not just on commit).

  -- Keir


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