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

["Shan, Haitao" <haitao.shan@xxxxxxxxx>]

Keir Fraser wrote:
> 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? 
In our problem, most of memory is allocate to dom0 by not specifying 
dom0_mem=xxx in grub. Dom0 actually has near 4G memory. So in this case, xen 
only has little memory below 4G, which is from the reservation pool.
> 
>  -- Keir
> 
> 
> 
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