PTE modified bit emulation trap

[John Baldwin]

On Thursday 14 May 2009 12:42:39 pm Alan Cox wrote:
> Dag-Erling Smørgrav wrote:
> > [from -alpha, -hackers]
> >
> > Dag-Erling Smørgrav <des at des.no> writes:
> >   
> >> Coverity complains about the lack of error checking in the following
> >> code in sys/kern/kern_sysctl.c, around line 1390:
> >>
> >> 	/*
> >> 	 * Touch all the wired pages to avoid PTE modified
> >> 	 * bit emulation traps on Alpha while holding locks
> >> 	 * in the sysctl handler.
> >> 	 */
> >> 	for (i = (wiredlen + PAGE_SIZE - 1) / PAGE_SIZE,
> >> 	    cp = req->oldptr; i > 0; i--, cp += PAGE_SIZE) {
> >> 		copyin(cp, &dummy, 1);
> >> 		copyout(&dummy, cp, 1);
> >> 	}
> >>
> >> Since Alpha is dead, can we remove this, or is it still needed for other
> >> platforms?
> >>     
> >
> > kmacy suggested you might be the right person to ask...  the conclusion
> > so far is that it *might* be necessary on sparc64 and / or mips.
> >   
> 
> I think that this code may no longer be needed, but I want to 
> double-check.  I faced a related problem implementing superpages 
> support, so I introduced an additional "access type" parameter to 
> pmap_enter().  This parameter was specifically intended to allow a 
> pmap_enter() implementation to preset the PTE's modified bit.  I think 
> that the simulated page fault that occurs on vslock()-style wiring 
> passes "write access" to pmap_enter().  If so, then it's just a matter 
> of tweaking the MIPS or any other pmap_enter() to actually do something 
> with the "access type" parameter.  Currently, only the architectures 
> that implement the pmap-level support for superpages, i.e., amd64 and 
> i386, do anything with this parameter.

Then it sounds like the code should definitely be removed and that if any 
problems do crop up, they can be fixed in pmap_enter() instead.

-- 
John Baldwin