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From: "John S. Dyson" <toor@dyson.iquest.net>
Message-Id: <199612151628.LAA05010@dyson.iquest.net>
Subject: Re: some questions concerning TLB shootdowns in FreeBSD
To: phk@critter.tfs.com (Poul-Henning Kamp)
Date: Sun, 15 Dec 1996 11:28:31 -0500 (EST)
Cc: peter@spinner.dialix.com, dyson@freebsd.org, smp@freebsd.org,
        haertel@ichips.intel.com
In-Reply-To: <9319.850645034@critter.tfs.com> from "Poul-Henning Kamp" at Dec 15, 96 11:17:14 am
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> 
> In message <199612151003.SAA14741@spinner.DIALix.COM>, Peter Wemm writes:
> >Poul-Henning Kamp wrote:
> >> In message <199612150121.JAA12763@spinner.DIALix.COM>, Peter Wemm writes:
> >> 
> >> >However, the shared address space code that I was working on in
> >> >-current (for kernel assisted threading in the smp kernel) means
> >> >that a single vmspace/pmap/etc can be shared among multiple processes
> >> >and this changes the above picture since two cpu's can be using
> >> >the user mode parts of the same page tables at once, one in executing
> >> >in user mode, one in the kernel.
> >> 
> >> But we could still have a per-cpu flags:
> >> 	"I'm not in a shared address-space"
> >> 
> >> Ie, this would only be set if the CPU was in userland in a non-threaded
> >> process.
> >
> >eg, something like: 
> >if (is_userland && curproc->p_vmspace->vm_refcnt > 1)
> >  send_tlb_invalidate();
> 
> Well more like:
> 
> 	for (i=0;i<NCPU;i++)
> 		if (is_userland(i) && curproc[i]->p_vmspace->vm_refcnt > 1)
> 			send_tlb_invalidate(i);
> 
This won't work because processes seldom have the entire address space
shared (vm_refcnt.)  I am sure that when we get true kernel multithreading
that will not be true though.  In order to test if a section of an
address space is shared, you have to do something like this (and
this can take LOTS of time.) (I might have levels of indirection
off here, I am also not taking into account submaps -- which
complicate the code further, by entailing recursively calling
the map/object traversal again -- but recursion is a major
no-no in the kernel, as we have found.)

	for(i=0;i<NCPU;i++) {
		tmpe = &curproc[i]->p_vmspace->vm_map->header.next;
		while (tmpe != &map->header) {
			if ((tmpe->is_sub_map == 0) && (tmpe->is_a_map == 0)) {
				tobj = tmpe->object.vm_object;
				while(tobj) {
					if (tobj->ref_count > 1) {
						send_tlb_invalidate(i);
						goto nextcpu;
					}
					tobj = tobj->backing_object;
				}
			} else {
				/*
				 * here on a new map -- need to recurse or
				 * somesuch here -- yuck!!!
				 */
			}
			tmpe = tmpe->next;
		}
nextcpu:
	}

John