> > What cross-cache copy?
> >
> > When we're talking really high performance, we're talking DMA and cards
> > that do the checksum on the card themselves.
> The headers that means, and for real network controllers (the ones that
> actually come in computers for normal people) the checksums.
AIX has to support certain microchannel fddi adapters which do not do
their own checksums. Last I counted, there are 5 read/writes getting the
data from DMA write to user space.
> No it means we need something akin to BSD m_pullup()
BSD MBUFs can really ruin your day if an IO device is doing pre-fetching
and the amount of actual data is a small fraction of a cache line. In the
case of a disk write, DMA prefetching can be a wonderful thing, really
makes disks fly. But it is not too cool in scatter-gather of short blocks
of data if your minimum prefetch is perhaps two or more cache lines.
As a side note, the 82450GX can look at a PCI originated DMA read and
based on the transfer info, prefetch on long blocks (e.g. disks) and not
prefetch on short blocks (e.g. ethernet). This is _so_ cool (and might
come in handy here).
Ed Welbon; welbon@bga.com;
<a href="http://www.bga.com/~welbon/arachnid.html">the arachnid mail list</a>