Re: [Xen-devel] A question about SHARE_PFN_WITH_DOMAIN

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 12 Jan 2006, at 08:20, Tian, Kevin wrote:

> I know the purpose of it is to share xenheap pages with domain, so with
> ownership change, type/count info update, etc. But is there any reason
> we can not allocate them from domheap instead, to grant ownership
> directly, since from the very start we know them shared between Xen and
> doman? Currently I just saw several types of pages are following this
> path:
>         - shared info page
>         - grant table
>         - trace buffer
>
> Maybe I missed some important hints about usage of domheap and xenheap
> there? ;-)
Well, there were originally two reason for allowing a guest to own 
xenheap pages:
Firstly, a domain page was only accessible in Xen/x86_32 with 
map_domain_page(), which is only intended for very short-term mappings. 
I added map_domain_page_global() as a temporary interface until current 
offenders who create long-term mappings of domheap pages are fixed. In 
fact, we could keep that as a permanent interface (if we created lots 
of domains we would probably run out of xen heap before we run out of 
domain_page_global mapping space).
Secondly, there is the subtle and thorny issue of domain destruction. 
Xen assume that any domain that has a non-zero reference count has a 
valid shared_info, for example. If we made shared_info a normal domheap 
page, and simply had Xen inc that page's refcnt like any other user, we 
would have a problem. Xen could not drop its reference until the domain 
refcnt drops to zero, but that won't happen until the domain owns no 
more pages!
We could work around that second issue with a new page flag to indicate 
pages that should not be freed to the heap when their refcnt drops to 
zero. Then Xen could set that flag instead of inc'ing the refcnt. That 
would allow the domain refcnt to fall to zero and Xen can then 
explicitly clear the flag and free the page in the domain destructor.
There's also the issue of whether these pages should be included in the 
domain's page count, and whether they should appear in its pseudo-phys 
space. If those issues get special-cased too, which they may need to 
for backward compatibility, the solution may end up as complex as the 
current xenheap-ownership solution.
 -- Keir


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