Re: [Xen-devel] Two question about grant table mechanism

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 19 Jun 2006, at 08:09, Lily Huang wrote:

> First question:
>
> If domA grant access permission of a page to DomB, and domB remapped 
> this page but then it crashes or failed to end mapping. Now domA wants 
> to end page granting, what will happen? Is there any way for page 
> owner to actively stop other VM's mapping?
No, it's expected you'll have an out-of-band mechanism for reclaiming 
the mapping, probably involving killing the offending DomB. This makes 
sense for the original usage scenario of driver domains, where if a 
backend driver has gone bad or got hung, merely getting back any 
current grants is unlikely to unstick things: you need a bigger hammer.
There's been discussion of other kinds of memory sharing mechanism 
which wouldn't have this problem -- a domain puts some memory to one 
side when it maps shared memory, which can then be revoked from under 
its feet by mapping the memory it put to one side in the shared 
memory's place. I think Rusty's patches implement something like this.
> Second question:
> If domA grant read-only permission on a page to domB, but domB tries 
> to write some data to this page. What will happen? General Memory 
> Protection?
DomB can only map the page read only, so write accesses will generate a 
page fault.
 -- Keir

> These two questions are very important to my research. Can someone 
> give me a answer to direct me somewhere so that I can read more 
> details about grant table.
> Many thanks!
> -lily
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