[Hackathon minutes] PV block improvements

["Roger Pau Monné" <roger.pau@citrix.com>]

Hello,

This are the notes about the block improvements discussed at the
Hackathon, some of them, if not all, have already been incorporated into:

https://docs.google.com/document/d/1Vh5T8Z3Tx3sUEhVB0DnNDKBNiqB_ZA8Z5YVqAsCIjuI/edit

Here is a list of future work items, more measurable and limited:

A) Separate request and response rings. This has several benefits, we
will be able to reduce the size of the response struct, since it no
longer has to be the same size of the request. Also, we could increase
the number of in-flight requests, since we are no longer limited by the
size of the request ring. We still need to make sure that all in-flight
requests can be written to the response ring once they are finished, or
added to a queue that writes them to the response ring when there's a
free slot.

B) Clean up the size differences between 32/64bit structs, also while
there reduce the size of a request so it is aligned to the cache (64bits).

C) Investigate the interrupt rate between blkfront/blkback and if needed
add support for polling, switching between polling or events could be
done automatically by blkfront/blkback when a high interrupt rate is
detected.

D) Multipage ring support.