On Sun, 21 Nov 1999 22:40:40 +0100 (MET) 
 Anders Magnusson <ragge@ludd.luth.se> wrote:

 > Now you came up with something else: PAGE_SIZE as a variable instead of
 > a constant create lots of unneccessary code, especially in complex
 > expressions that otherwise could be optimized away by the compiler.
 > What is the reason to have it as a variable instead of a constant?
 > Can there ever be a benefit from this?

"Because that's the way Mach VM hid MI vs. MD page size"?  :-)

Seriously, though, I think that what we should do is change the following
snippet of code in vm/vm_param.h from:

#if defined(_KERNEL)
/*
 *	All references to the size of a page should be done with PAGE_SIZE
 *	or PAGE_SHIFT.  The fact they are variables is hidden here so that
 *	we can easily make them constant if we so desire.
 */
#define	PAGE_SIZE	uvmexp.pagesize		/* size of page */
#define	PAGE_MASK	uvmexp.pagemask		/* size of page - 1 */
#define	PAGE_SHIFT	uvmexp.pageshift	/* bits to shift for pages */
#endif /* _KERNEL */

to:

#if defined(_KERNEL) && !defined(PAGE_SIZE)
/*
 *	All references to the size of a page should be done with PAGE_SIZE
 *	or PAGE_SHIFT.  Machine dependent code may override these definitions,
 *	making them constants.  Systems with page size detected at run-time
 *	should use these definitions instead.
 *
 *	Note: these variables should always be initialized even if constants
 *	are used.  This is for the benefit of user programs which want to
 *	read the `uvmexp' structure.
 */
#define	PAGE_SIZE	uvmexp.pagesize		/* size of page */
#define	PAGE_MASK	uvmexp.pagemask		/* size of page - 1 */
#define	PAGE_SHIFT	uvmexp.pageshift	/* bits to shift for pages */
#endif /* _KERNEL && ! PAGE_SIZE */

#if !defined(PAGE_SIZE) || !defined(PAGE_MASK) || !defined(PAGE_SHIFT)
#error PAGE_SIZE, PAGE_MASK, and PAGE_SHIFT must be defined
#endif

...then, <machine/vmparam.h> could do what it sees fit.

        -- Jason R. Thorpe <thorpej@nas.nasa.gov>