PTE modified bit emulation trap

Alan Cox alc at
Thu May 14 16:42:48 UTC 2009

Dag-Erling Smørgrav wrote:
> [from -alpha, -hackers]
> Dag-Erling Smørgrav <des at> 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.


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