swapping/paging and scatter-gather

[Chris Torek]

In article <6936 at june.cs.washington.edu> ka at 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 at mimsy.umd.edu	Path:	uunet!mimsy!chris