Path: utzoo!attcan!uunet!lll-winken!ames!haven!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.unix.wizards
Subject: swapping/paging and scatter-gather
Keywords: swapping
Message-ID: <15457@mimsy.UUCP>
Date: 13 Jan 89 12:37:29 GMT
References: <627@cimcor.mn.org> <6936@june.cs.washington.edu>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 17

In article <6936@june.cs.washington.edu> ka@june.cs.washington.edu
(Kenneth Almquist) writes:
>... on some machines, devices can only do I/O to contiguous regions
>of physical memory. ...

The Tahoe has some devices that act like this.  The `solution' used
in 4.3BSD is to have a few hundred kbytes of physically contiguous
memory allocated to the vba (Versabus/VMEbus adapter) code, and to
copy the data in and out.  Gross, but it works; the vba allocation
code is careful to figure out whether this sort of munging is really
necessary.  I had to do something similar in the BI KDB driver,
which reads through hardware PTEs; it needs physically contiguous
PTEs even in user space, and they are not available.  (Fortunately,
>80% of the transfers done in the system run through the buffer cache.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris