Using splice to carry Xen granted pages through the network stack?

From: Jeremy Fitzhardinge
Date: Wed Mar 18 2009 - 20:05:17 EST


Hi Jens,

In a thread from December last year ("Support for zero-copy TCP transmit of user space data") you suggested using the splice machinery as a way to implement zero-copy transmit for iscsi data.

I have a similar problem relating to doing zero-copy transmit of pages granted from another Xen domain. These are pages which are in some ways similar to device mappings (they have no struct page unless we jump through some hoops to give them one, for example) which are given to us by another domain (virtual machine)'s network stack since they contain data they want to transmit through our stack. Once the stack has finished with the pages we need to get them back to return to the original domain (and definitely not let them get freed into the normal kernel pool).

I've been looking a little bit at the splice stuff to work out how it might be useful in this case. One issue struct page; I guess we really can't get away from having one for granted pages and still get the full value of using splice (especially I can see also see it being useful on the block side of the world as well). So, we can jump through hoops for that.

But the general plumbing of splice into the network stack seems to have been an open issue since you introduced it 3 years ago (using as a reference http://lwn.net/Articles/181169/). Has any work been done on this aspect since? What would need to be done to do it? Would it mean wiring up a pipe_buffer_operations to operate with something like Rusty's skb_shared_info destructor?

Thanks,
J
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