From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <3C15E692.18FA79CB@strakt.com> From: Boyd Roberts MIME-Version: 1.0 To: 9fans@cse.psu.edu Subject: Re: [9fans] RDMA: DMA over TCP/IP at IETF References: <200112102218.IAA08394@hadrian.staff.apnic.net> <3C15925F.73746F2A@null.net> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Date: Tue, 11 Dec 2001 11:57:22 +0100 Topicbox-Message-UUID: 32a74e82-eaca-11e9-9e20-41e7f4b1d025 "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].