From owner-freebsd-questions  Wed Oct 24 11:23:32 2001
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Date: Wed, 24 Oct 2001 13:22:07 -0500 (CDT)
From: mark tinguely <tinguely@web.cs.ndsu.nodak.edu>
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To: freebsd-questions@FreeBSD.ORG, xuyifeng@peoplemail.com.cn
Subject: Re: A stupid question about pmap
In-Reply-To: <001b01c15c27$2332db60$ef01a8c0@davidwnt>
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>      unsigned frame = (unsigned) pmap->pm_pdir[PTDPTDI] & PG_FRAME;
>
>      /* are we current address space or kernel? */
>      if (pmap == kernel_pmap || frame == (((unsigned) PTDpde) & PG_FRAME)){
>
>  I know pm_pdir[PTDPTDI] contains page directory page's physical address, 
>  while PTDpde is a virtual address to access page directory's page directory 
>  entry which is recursivly mapped on virtual address space.

pm_pdir[] entries are in kernel virtual address. see how it is being
loaded/initialize, for example the kernel's pmap is loaded:
	kernel_pmap->pm_pdir = (pd_entry_t *) (KERNBASE + (u_int)IdlePTD);

or more obvious:
	pmap->pm_pdir =
		(pd_entry_t *)kmem_alloc_pageable(kernel_map, PAGE_SIZE);

and when we need to load the CR3 register, it gets turns it into a
physical address:
	load_cr3(p->p_addr->u_pcb.pcb_cr3 = vtophys(pmap->pm_pdir));

This routine is simply checking to see this is the kernel map, or if the
address space is the current PTD, if not we need to load the PTD into the
Alternative PTD.

--mark.

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