Re: [Xen-devel] Re: Next steps with pv_ops for Xen

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 5/12/07 17:48, "Derek Murray" <Derek.Murray@xxxxxxxxxxxx> wrote:

> Keir Fraser wrote:
>> Need to bite the bullet and fix this properly by setting a software flag in
>> ptes that are not subject to reference counting.
> 
> Could we get away with testing the VM_FOREIGN flag in vm_normal_page()?
> Although I get the impression that this wouldn't be easily justified if
> trying to merge with upstream Linux....

Yes, this would work okay I suspect. Good enough as a stop-gap measure? Are
there any other responsibilities that you acquire if you make use of
VM_FOREIGN (in particular, how would this affect get_user_pages)?

> Alternatively, could we use the _PAGE_GNTTAB PTE flag that is used for
> debugging? Indeed, if we did this, could be obviate the need for the
> PTE-zapping hook, by instead catching the case where this flag is set,
> and unmapping the grant implicitly?

Well, in the general case you don't have enough info to know which grant to
release (a single page can be granted multiple times).

> Otherwise, what would the semantics of this new flag be?

It would cause pte_pfn() to return max_mapnr. It would be set for any
foreign page mapping, and replace mfn_to_local_pfn() in pte_pfn().

 -- Keir



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