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Date: Mon, 07 Jun 2010 16:29:27 -0500
From: Alan Cox <alc@cs.rice.edu>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: "Jayachandran C." <jchandra@freebsd.org>, mips@freebsd.org
Subject: Re: svn commit: r208589 - head/sys/mips/mips
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On 6/7/2010 3:28 PM, Kostik Belousov wrote:
> Selecting a random message in the thread to ask my question.
> Is the issue that page table pages should be allocated from the specific
> physical region of the memory ? If yes, doesn't i386 PAE has similar
> issue with page directory pointer table ? I see a KASSERT in i386
> pmap that verifies that the allocated table is below 4G, but I do not
> understand how uma ensures the constraint (I suspect that it does not).
>    

For i386 PAE, the UMA backend allocator uses kmem_alloc_contig() to 
ensure that the memory is below 4G.  The crucial difference between i386 
PAE and MIPS is that for i386 PAE only the top-level table needs to be 
below a specific address threshold.  Moreover, this level is allocated 
in a place, pmap_pinit(), where we are allowed to sleep.

Alan