Re: xenheap pages mapped by dom0

[George Dunlap <george.dunlap@eu.citrix.com>]

Jan Beulich wrote:
>>>> Keir Fraser <keir.fraser@eu.citrix.com> 18.12.09 16:24 >>>
>>>>         
>> I think the right thing to do would be very roughly:
>> On allocation:
>> page = alloc_xenheap_page();
>> share_xen_page_with_guest(page);
>> On deallocation:
>> if (test_and_clear(PGC_allocated)) put_page();
>> if (page->count_info & PGC_count_mask) return -EBUSY;
>>     
>
> Hmm, and how would you recover from that? I don't think George wants
> his enclosing operation to fail because of a page still being mapped. On
> x86-64, at least, it might be possible to play with PGC_xen_heap, to
> convert a Xen heap page to a domain heap one, but that wouldn't
> cover anyone else (and would seem rather hackish, if it works at all).
>   
I wouldn't really mind the op failing; I just want it to fail cleanly. 
All-or-nothing would be optimal IMHO.  Failing such that everything will 
automatically clean itself up if/when dom0 does unmap the pages would be 
OK too.  Having half the pages freed / marked unallocated, but some 
still allocated doesn't seem very good.

Anyway, I care more about allocating larger buffers than I do about 
re-allocating, so maybe I'll just punt on freeing maybe-mapped buffers 
for now.

However, that brings up another point: suppose that I do several 
allocations, one per cpu, and one fails.  Now I want to go and free the 
buffers I've just allocated (which shouldn't be mapped in dom0 yet).   
Can I not just call free_xenheap_pages()? Do I need to clear 
PGC_allocated before calling?

Is any of this stuff written down somewhere? :-)

 -George