[Xen-devel] Mapping another domain's user memory

[Michael Abd-El-Malek <mabdelmalek@xxxxxxx>]

Hello,

Is it possible to map another domain's user-level memory into a  
different domain's kernel?  (For clarity, let the source domain be the  
one with the user-space pages that a destination domain wants to write  
to.)
I tried to do that by first getting a grant for the user-space memory  
in the source domain as follows:
        page_start = user_buf;
        buffer_mfn = virt_to_mfn(page_start);
	gnttab_grant_foreign_access_ref(grant_ref, info->dev->otherend_id,  
buffer_mfn, 0);
But the virt_to_mfn call asserted, in include/asm-x86_64/mach-xen/asm/ 
maddr.h:pfn_to_mfn.
The assertion that failed was: BUG_ON(end_pfn && pfn >= end_pfn);

So a few questions:
1) is it even possible to map a domain's user-space page into another  
domain?
2) for the above code snippet to work, must I install some mappings in  
the source domain first?
3) does the blktap driver do something similar? I tried looking at the  
blktap driver, because it looked like it might help.  But I'm new to  
the Xen memory management code so it was a bit difficult to follow.
4) I looked at fs/filemap.c:file_read_actor and how it calls kmap  
before calling __copy_to_user.  Would calling kmap help?   
__copy_to_user is a bit of a mystery to me (granted, no pun intended,  
I didn't dig deep enough into the assembly code to understand what  
it's doing.)
5) any other suggestions?

Thanks,
Mike

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