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RE: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback

> The only catch is that I wanted to have the same lazy-copy
> mechanism for dom0=>domU.  In that case we can't rely on domU
> to trigger the copy so dom0 (or the driver domain) will need
> to initiate it.  The easiest way to do that from outside domU
> is through changing its P2M table.
> I suppose doing it this way doesn't necessarily preclude a
> different solution for dom0=>domU.

I think we can make the guest TX path work rather better by putting the
tx packet header off into a separate fragment and placing it within one
of a set of pages that are 'permanently' mapped between the front and
back ends. The backend would not typically grant map the payload
fragment(s), but only do so if it catches a page fault trying to access

The way I'd do the 'permanent' mappings is to have a bit in the grant
reference passed across the ring that indicates that the backend is free
to leave the grant mapped. The bit should be set whenever the grant
entry is used (the front end can't mix and match). The backend thus just
needs to keep track of which grant refs it has mapped via this permanent
scheme so that it can spot (valid) reuse, and clean up afterwards. Since
the front end will typically use a block of grant refs for this purpose
the backend code can be optimised for a simple range check. The frontend
is free to pack multiple header fragments on to a page to save memory
(rather than using larger payload sized frags).

Jose Renato investigated something similar to this last year, but AFAIK
we didn't take it the whole way of introducing permanent mappings with
no (default) mapping of payloads. 

BTW: when adding fragment support you 'borrowed' the descriptor id field
which wasn't being used at the time.  This means we can no longer return
free buffers to the rx ring out of order. This is rather unfortunate as
its very useful for dealing with some kinds of smart NIC. I expect we'll
have to introduce a net ring v2 format in the not too distant future
unless we can think of a cunning way of stealing them back?


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