linux-kernel - Re: [PATCH] x86: Construct 32 bit boot time page tables in native format.

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Date:	Mon, 21 Jan 2008 13:38:22 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ian Campbell <ijc@...lion.org.uk>
CC:	Mika Penttilä <mika.penttila@...umbus.fi>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] x86: Construct 32 bit boot time page tables in native
 format.

Ian Campbell wrote:
> On Sun, 2008-01-20 at 20:30 +0200, Mika Penttilä wrote:
>>>   
>> You have dropped the requirement to map all of low memory (the boot 
>> allocator is used for instance to construct physical mem mapping). 
>> Either you should fix your INIT_MAP_BEYOND_END or make a big comment 
>> telling us why it isn't necessary anymore to map low mem.
> 
> I think you are right. The patch ensures that all the initial page
> tables themselves have mappings but won't map the additional pages
> needed for mapping the rest of lowmem.
> 
> However, I think it is no longer necessary to map a whole new 4G worth
> of page table pages because the code in kernel_physical_mapping_init now
> extends the initial mappings rather than replacing them (see changes to
> native_pagetable_setup_start). So now we only need to map 4G worth of
> page tables including the initial page tables. That means we only need
> to map a fixed set of extra pages rather than the sliding limit
> currently used in the patch.
> 

We still need to be able to construct those page tables, which is what 
that stuff is about...


> I'm not convinced by the additional 16MB for CONFIG_DEBUG_PAGEALLOC --
> we map enough pages for page tables for 4G of lowmem -- adding space for
> an extra 16M seems pointless.

If so, adjusting the limit should be a separate patch.

Either way, I'm increasingly thinking that setting up the initial page 
tables via an assembly loop instead of worrying about the C accessors is 
actually cleaner (I prototyped it yesterday, although I still need the 
rest of the machinery.)


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