Re: [9fans] RDMA: DMA over TCP/IP at IETF

[Boyd Roberts <boyd@strakt.com>]

"Douglas A. Gwyn" wrote:
> many times, e.g. on some variants of Unix
> there were hacks so that a user-mode buffer
> that an application arranged to be page-
> aligned would be directly mapped onto the
> kernel address space so DMA (for example)
> would directly get/put the data where the
> application wanted it.  It would of course
> be much better if this didn't have to be
> done as a hack.

Some versions for the VAX mapped the UNIBUS address space
smack on top on the clists so if you had tty h/w that could
DMA (the DZ-11? or did you need KMC's for that?) you wouldn't
have to map the UNIBUS for each DMA.  This was a grody hack,
but the alternatives were just too slow, particularily if
you wanted to support 128 students on a '780.

Mapping it into user mode is a real can of worms;  not only is
it highly unportable it really messes up the memory management
because pages have to be locked.  Locking is a nightmare;  don't
lock and you lose, have too many locks and horrible bugs appear
either immediately [bad] or months/years later [worse].