Re: [Xen-devel] Mapping pages from dom0

[John L Griffin <jlg@xxxxxxxxxx>]

> I am mapping some pages in a domU from dom0 userspace. Doing that once I 

> know the page numbers is fairly trivial, but what is the recommended way 

> of communicating one or more page numbers from domU to dom0, hopefully 
> without having to modify xend?

I am curious: I'm just starting to look into the mechanisms by which pages 
are shared between domains.  The above question surprised me, because I'd 
assumed there existed some sort of anonymous "I want to map a page that 
domain N has exported" method to bootstrap direct interdomain 
communication between unprivileged domains.

Looking through the code now, could someone please comment on the 
correctness (or, more importantly, uncorrectness) of the following 
statements?

* It is possible for two unprivileged domains to share pages, without 
needing any intervention from a privileged domain, as long as each side 
knows in advance the addresses of the pages to be shared?

* The grant_table_op hypervisor call is used by Domain-A to notify Xen 
that "yes, I want to allow Domain-B to map this page"?  Then, the 
update_va_mapping hypervisor call is used by Domain-B to map the page?

* grant_table_op cannot be used by Domain-A to revoke Domain-B's mapping 
of the page?  (I.e., all the "unmap grant ref" flag says is "cause all 
future Domain-B update_va_mapping calls, regarding mapping this page, to 
fail"?)

* Jacob's problem (dom0 needing to know the addresses of pages shared by a 
domU) is avoided during virtual network and virtual block device setup, 
because the pages to be shared are somehow either set up by Xen or shared 
through the "control interface" channel to Xend?

Thanks,
JLG



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