[Xen-devel] Re: [Question] Why code differs in construct_dom0?

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/11/08 09:41, "Shan, Haitao" <haitao.shan@xxxxxxxxx> wrote:

> So you mean in the release build we make the mapping discontiguous to detect
> possible bugs, while in debug build it is not discontiguous?

It's the other way round.

> And another question from problems we encountered recently, system with more
> than 4G memory installed will crash when X server shutdown. The reason is:
> 1> dom0 allocates memory for agp by calling agp_allocate_memory with GFP_DMA32
> set. This implies the pfn comes from memory lower than 4G, while mfn are
> likely to be from memory above 4G.
> 2> dom0 then call map_pages_to_apg, since the kernel of handles 32bit gart
> table, dom0 uses hypercall to change its memory mappings
> (xen_create_contiguous_region). Xen will pick proper memory below 4G and free
> those from the guest (likely to be from memory above 4G).
> 3> As the process goes on. More and more memory below 4G is return to dom0
> while leaving memory above 4G in xen. Finally, xen's reservation of memory
> below 4G for DMA are exhausted. This creates severe problems for us.
> 
> What is your comments on this? Both increase the reservation in Xen and using
> contiguous mappings are helpful in this cases. Which one do you prefer?

I'd need more info on the problem. I will point out that 64-bit Xen only
allocates memory below 4G when asked, or when there is no memory available
above 4G. Actually 32-bit Xen is the same, except the first chunk of dom0
memory allocated has to be below 1GB (because of limitations of Xen's
domain_build.c). So I'm not sure what more Xen can do?

 -- Keir



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