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

H. Peter Anvin
2008-01-21T21:39:23Z

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.)