Re: Fw: [Xen-devel] Xen on /. again

[Steven Hand <Steven.Hand@xxxxxxxxxxxx>]

>
>>Mark Williamson wrote:
>>>>Also, I suppose you will wish to prevent covert channels between
>>>>domains, e.g. domains communicating using various timing attacks (I move
>>>>the disk head to the other end of the disk if I wish to tell you
>>>>something), or by allocating/freeing certains parts of memory.
>>>>
>>>>How much will you need to dumb down the VMs view of what is going on in
>>>>the machine to achieve this (not expose real time information, not
>>>>expose real page tables), and how much of a VMM will there be left when
>>>>you are done?
>>> 
>>> Well domains are not aware of each other's memory usage, so I wouldn't have
 
>>> thought that allocation / exposing real page tables would matter.  (Except 
>>> dom0 can of course see everything if it wants).
>>
>>Information about other domains' memory usage is leaked via the
>>hardware->physical mapping.
>
>Unprivileged domains cannot see each others hardware->physical mappings. 
 
Oops - I read this originally as 'virtual->physical'. The MPT does indeed
currently allow domains to view each others hardware->physical mappings. 
Replacing the single MPT with per-domain viewable sparse mapping structures 
is possible though, albeit at a performance cost.  Full virt with VT-x also 
eliminates this issue.

cheers, 

S.



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