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

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

I forgot to mention in our configuration we did not specify the dom0_mem. Dom0 
was allocated a large portion of the total system memory.

Best Regards
Haitao Shan

Shan, Haitao 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? 
> 
> 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?  
> 
> Best Regards
> Haitao Shan
> 
> Keir Fraser wrote:
>> By deliberately making dom0's p2m mapping discontiguous we can detect
>> bugs where dom0 is incorrectly assuming pseudophys contiguous memory
>> is machine contiguous. We had nasty bugs of this sort in dom0's
>> block layer many years ago. 
>> 
>>  -- Keir
>> 
>> On 20/11/08 09:07, "Shan, Haitao" <haitao.shan@xxxxxxxxx> wrote:
>> 
>>> Hi, Keir,
>>> 
>>> Please see the code shown below. I don't understand why pfn's
>>> definitions are different depending on whether NDEBUG is defined or
>>> not. Can you tell me why? 
>>> 
>>>     while ( pfn < nr_pages )
>>>     {
>>>         if ( (page = alloc_chunk(d, nr_pages - d->tot_pages)) ==
>>>             NULL ) panic("Not enough RAM for DOM0 reservation.\n");
>>>         while ( pfn < d->tot_pages )
>>>         {
>>>             mfn = page_to_mfn(page);
>>> #ifndef NDEBUG
>>> #define pfn (nr_pages - 1 - (pfn - (alloc_epfn - alloc_spfn)))
>>>             #endif if ( !is_pv_32on64_domain(d) )
>>>                 ((unsigned long *)vphysmap_start)[pfn] = mfn;
>>>                 else ((unsigned int *)vphysmap_start)[pfn] = mfn;
>>>             set_gpfn_from_mfn(mfn, pfn);
>>> #undef pfn
>>>             page++; pfn++;
>>>         }
>>>     }
>>> 
>>> Best Regards
>>> Haitao Shan
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