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Re: [Xen-devel] [PATCH 2/2] x86/hvm: honor guest's option when updating secondary system time for guest

>>> On 25.07.14 at 10:52, <andrew.cooper3@xxxxxxxxxx> wrote:
> Thinking about this some more, how can we possibly know whether writing
> to the nominated page is even safe?  At the point of update, there is no
> knowledge about the current cr3 in use.

That's a problem the kernel has to be bothered by, but not the

> As far as I can see, the only safe actions Xen can take is refuse to
> ever write to user mappings, and insist that any OS using this feature
> provide a supervisor mapping to Xen (and guarantee it will never swap
> the target of that mapping) and give user mappings of the same gpa to
> any of its own userspace which wants access.  This then avoids any
> concerns wrt SMAP.

No, user mappings can validly (and do) appear in the non-user
mode parts of the page tables.


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