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Re: [Xen-devel] Questions about xen memory management

On Thu, 2013-08-08 at 14:59 +0800, Josh Zhao wrote:
> Hi,
> I am reading the arm MM initial code, there are 2 questions I can't
> understand:
> 1)  Both init_xenheap_pages() and init_domheap_pages() will invoke
> init_heap_pages() to initialize pages management.  But there is no
> flag to know those pages are belonged to xenheap or domheap.  Are
> xenhelp and domheap in the same zone?

There are two models for xen vs. domheap, and therefore two version of

The original model is the split heap model, which is used on platforms
which have smaller virtual address spaces. e.g. arm32, for the moment
arm64 (but I am about to switch to the second model) and historically
the x86_32 platform. This is because as Andy notes xenheap must always
be mapped while domheap is not (and cannot be on these platforms),
domheap is mapped only on demand (map_domain_page).
In this case init_xenheap_pages contains:
     * Yuk! Ensure there is a one-page buffer between Xen and Dom zones, to
     * prevent merging of power-of-two blocks across the zone boundary.
    if ( ps && !is_xen_heap_mfn(paddr_to_pfn(ps)-1) )
        ps += PAGE_SIZE;
    if ( !is_xen_heap_mfn(paddr_to_pfn(pe)) )
        pe -= PAGE_SIZE;

The second model is used on systems which have large enough virtual
address to map all of RAM, currently x86_64 and soon arm64. In this case
there is only one underlying pool of memory and the split is more
logical than real, although it is tracked by setting PGC_xen_heap when
allocating xenheap pages. In this case domheap is actually always mapped
but you still must use map_domain_page to access it (so common code work
on both models)

There is actually an extension to the second model for systems which
have enourmous amounts of physical memory (e.g. >5TB on x86_64) which
brings back the xen/domheap split but in a different way to the first
model. In this case the split is implemented in alloc_xenheap_pages by
consulting xenheap_bits to restrict the allocations to only the direct
mapped region.

> 2) What's the vmap.c used for ?

To map arbitrary physical addresses into the virtual address space.

>  I saw that only the ioremap will use it.  If so, it seems no needs to
> allocate pages to fill the all VMAP range (256M -  1G) by
> alloc_domheap_page().

This is allocating the page table pages up front which simplifies the
creation of mappings. In theory this could be done on demand but I
suppose it is simpler to do it up front.


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