Difference between alloc_domheap_pages vs. alloc_xenheap_pages?

Difference between alloc_domheap_pages vs. alloc_xenheap_pages?

[Xinxin Jin]

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Hi,

I noticed the only difference between the two is to add PGC_xen_heap flag
in allocated xenheap pages. So does it matter to exchange these two
functions when allocating a heap page?

Thanks,
Xinxin

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Re: Difference between alloc_domheap_pages vs. alloc_xenheap_pages?

[Ian Campbell]

On Wed, 2013-06-26 at 15:41 -0700, Xinxin Jin wrote:


> I noticed the only difference between the two is to add PGC_xen_heap
> flag in allocated xenheap pages. So does it matter to exchange these
> two functions when allocating a heap page?

xenheap pages are always mapped, domheap pages are only mapped on demand
with (un)map_domain_page. The big clue is that alloc_xenheap_pages
returns a void* while alloc_domheap_pages returns a struct page_info*.

On some architectures with a large virtual address space (i.e. 64-bit
ones) the two can be combined and the distinction becomes a bit moot,
but it remains for the benefit of common code and also because even on
64-bit architectures when you have large amounts of RAM you may still
end up with RAM which is not permanently mapped -- e.g. on x86_64 if you
have >5TB of RAM then you end up with a split again.

Ian.

Re: Difference between alloc_domheap_pages vs. alloc_xenheap_pages?

[Keir Fraser]

On 27/06/2013 09:11, "Ian Campbell" <Ian.Campbell@citrix.com> wrote:

> On Wed, 2013-06-26 at 15:41 -0700, Xinxin Jin wrote:
> 
> 
>> I noticed the only difference between the two is to add PGC_xen_heap
>> flag in allocated xenheap pages. So does it matter to exchange these
>> two functions when allocating a heap page?
> 
> xenheap pages are always mapped, domheap pages are only mapped on demand
> with (un)map_domain_page. The big clue is that alloc_xenheap_pages
> returns a void* while alloc_domheap_pages returns a struct page_info*.
> 
> On some architectures with a large virtual address space (i.e. 64-bit
> ones) the two can be combined and the distinction becomes a bit moot,
> but it remains for the benefit of common code and also because even on
> 64-bit architectures when you have large amounts of RAM you may still
> end up with RAM which is not permanently mapped -- e.g. on x86_64 if you
> have >5TB of RAM then you end up with a split again.

Also xenheap-allocated pages must be explicitly freed by Xen, usually during
domain destruction.

> Ian.
> 
> 
> 
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