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

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Fri, 2013-08-09 at 11:15 +0800, Josh Zhao wrote:
> Thanks for your reply.
> 
> 2013/8/8 Ian Campbell <Ian.Campbell@xxxxxxxxxx>
>         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
>         init_*heap_pages.
>         
>         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;
>         
> 
> 
> Yes, is_xen_heap_mfn() function can whether the physical address is in
> xenheap or not.  But how the init_heap_pages() knows the page is
> belonged to xenheap or domheap. Because the phys_to_nid()  always
> return 0;

This is on ARM?

On ARM xenheap pages can be identified by comparing the mfn to
xenheap_mfn_{start,end} -- the xenheap is always a contiguous range of 
memory.

phys_to_nid gets you the NUMA node ID which is mostly orthogonal to
xenheap vs domheap.

> As setup_xenheap_mappings() function is one to one mapping  xenheap
> physical to 1G--2G virtual space range. I am wondering what the
> Xenheap virtual space(1G-2G) is used for ?   Because I can allocate
> xen pages from xenheap by alloc_xenheap_pages()  and I can track pages
> by frametable.

I'm not sure what you are asking here. The 1G--2G virtual mapping of the
xenheap exists exactly to satisfy the requirement that xenheap pages are
always mapped IOW __pa and __va work only on xenheap pages.

Sure, you could take a xenheap page, extract the mfn and use
map_domain_page, but why?

>         > 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.
> 
> 
> 
> 
> In VM_init():
> 
> 
> for ( i = 0, va = (unsigned long)vm_bitmap; i < nr; ++i, va +=
> PAGE_SIZE )
>     {
>         struct page_info *pg = alloc_domheap_page(NULL, 0);
> 
> 
>         map_pages_to_xen(va, page_to_mfn(pg), 1, PAGE_HYPERVISOR);
>         clear_page((void *)va);
>     }
> 
> 
> It seems not only allocate page table pages ,but also allocates vmap
> pages by alloc_domheap_page().

I'm not sure what you are asking here either.

This is populating the vmap virtual address space, not the xenheap
virtual address space, so it allocates a domheap page and then maps it
explicitly in the appropriate place.

Ian



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