Re: [Xen-devel] Uses of &frame_table[xfn]

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 23 Dec 2005, at 13:28, Magenheimer, Dan (HP Labs Fort Collins) wrote:

> Ah, I see.  For unprivileged domains, this assumption is also valid
> on ia64 (though from Ewan's reply, I gather this is is a no-op
> for unprivileged domains because max_pfn==nr_pages at launch?).
Very early on in boot we increase max_pfn beyond nr_pages and allocate 
a suitably large mem_map so that there are struct page's covering the 
whole space.
> > the physical space between nr_pages and max_pfn is not (yet)
> > populated with RAM
> How do you manage that? Does "not populated" mean there is no
> struct page for them?  Will a get_free_page work on them?
> We just tracked down a horrific problem involving missing
> dom0 struct pages on Xen/ia64 so I want to make sure I understand
> this and get it right.
Yeah, they have struct page, but there is no memory mapped at that 
kernel virtual address (the phys2mach entry points nowhere).
> If there is a struct page and the Linux allocator can handle
> the pages, my previous suggestion of replacing the code with
> alloc'ing N pages should work for both (x86 and ia64).  If not,
> how do you feel about replacing that memory initialization
> stanza with a call to arch_balloon_init (and creating a new
> asm/balloon.h file)?
Alloc'ing N pages would not guarantee to get exactly the pages we want 
(i.e., the ones with no RAM). The mem initialization as is does not 
look particularly x86 specific. Is the check for PageReserved a 
problem? If so, we can probably work around that -- it's only used to 
skip over highmem pages in non-highmem i386 kernels.
 -- Keir


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