[Projectudi-tech] Re: [UDI-Tech] Copy avoidance in UDI?

[Robert L. <ro...@sc...>]

Deven T. Corzine wrote:

> zero-copy network stacks (for example) should be possible with UDI,
> yet it's far from obvious exactly how that would work.

It seldom is.

> equally high performance as a native driver interface?  How could a
> zero-copy design be implemented with UDI, exactly?  (Would zero-copy

Because the buf path is passed all the way up and down the I/O path, the
VM system can provide an optimal mapping that satisfies the alignment
concerns of the hardware ("I can't DMA to anything other than even
numbered pages") the upper layers ("the IP address must have alignment
of X") and everything in the middle.

If you stack the deck badly enough against any system, you will have
to copy the data at some point.  You won't get zero copy from, say,
a dual-ported RAM card.  Fortunately, broken (or non-existent) DMA
controllers are more rare than they used to be.

RJL
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