From owner-freebsd-mips@FreeBSD.ORG  Thu Aug  5 06:13:56 2010
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Date: Thu, 05 Aug 2010 01:13:47 -0500
From: Alan Cox <alc@cs.rice.edu>
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To: "Jayachandran C." <jchandra@FreeBSD.org>
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Cc: mips@freebsd.org
Subject: Re: svn commit: r210846 - in head/sys/mips: include mips
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Just an observation ...

Jayachandran C. wrote:
> Author: jchandra
> Date: Wed Aug  4 14:12:09 2010
> New Revision: 210846
> URL: http://svn.freebsd.org/changeset/base/210846
>
> Log:
>   Add 3 level page tables for MIPS in n64.
>   
>   - 32 bit compilation will still use old 2 level page tables
>   - re-arrange pmap code so that adding another level is easier
>   - pmap code for 3 level page tables for n64
>   - update TLB handler to traverse 3 levels in n64
>   
>   Reviewed by:	jmallett
>   
[snip]
>  
> -		if (pbits != *pte) {
> -			if (!atomic_cmpset_int((u_int *)pte, obits, pbits))
> -				goto retry;
> -			pmap_update_page(pmap, sva, pbits);
> +			/* Skip invalid PTEs */
> +			if (!pte_test(pte, PTE_V))
> +				continue;
> +retry:
> +			obits = pbits = *pte;
> +			pa = TLBLO_PTE_TO_PA(pbits);
> +			if (page_is_managed(pa) && pte_test(&pbits, PTE_D)) {
> +				m = PHYS_TO_VM_PAGE(pa);
> +				vm_page_dirty(m);
> +				m->md.pv_flags &= ~PV_TABLE_MOD;
> +			}
> +			pte_clear(&pbits, PTE_D);
> +			pte_set(&pbits, PTE_RO);
> +			
> +			if (pbits != *pte) {
> +				if (!atomic_cmpset_int((u_int *)pte, obits, pbits))
> +					goto retry;
> +				pmap_update_page(pmap, sva, pbits);
> +			}
>  		}
>   

MIPS doesn't really need to use atomic_cmpset_int() in situations like 
this because the software dirty bit emulation in trap.c acquires the 
pmap lock.  Atomics like this appear to be a carryover from i386 where 
the hardware-managed TLB might concurrently set the modified bit.