I've just committed some changes to the sun3x code to
allow building a UVM kernel, except for one thing:
It needs to be switched to MACHINE_NEW_NONCONTIG
before UVM will compile.  The patch below will enable
that, but I want to give it some more testing before
commiting the change.  Feel free to try it out...

*** sys/arch/sun3/include/vmparam3x.h~	Thu Feb  5 14:06:30 1998
--- sys/arch/sun3/include/vmparam3x.h	Tue Jun  9 16:17:56 1998
***************
*** 106,109 ****
--- 106,121 ----
  #define VM_KMEM_SIZE		(NKMEMCLUSTERS*CLBYTES)
  #define VM_PHYS_SIZE		(USRIOSIZE*CLBYTES)
  
+ #if 0
  #define MACHINE_NONCONTIG	/* VM <=> pmap interface modifier */
+ #else
+ #define MACHINE_NEW_NONCONTIG	/* VM <=> pmap interface modifier */
+ 
+ #define VM_PHYSSEG_MAX		4
+ #define VM_PHYSSEG_STRAT	VM_PSTRAT_BSEARCH
+ #define VM_PHYSSEG_NOADD	/* can't add RAM after vm_mem_init */
+ 
+ struct pmap_physseg {
+ 	/* NULL */
+ };
+ #endif