Re: Never mind Re: [Xen-devel] Re: Xen reboots when trying to start new domain

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> When do the changes to the ptes take effect? How does Xen enforce 
> security in this case? My guess would be to detach the leaf table from 
> the page directory, and reattach and check it on demand at first fault. 
> Is this correct? Does Xen keep shadow copies of all pgds then, or only 
> the currently active one? Sounds clever.

You just described version 1 of writable pagetables. In v2, when the
guest tries to write his pagetable we instead give him write access to
a copy of that table. Then, next time he TLB flushes, or the guest
faults on the VA area that th eupdated p.t. maps, we validate any
modifications and copy them back into the real p.t. This means there
is no need to unlink the real p.t. at any point.

 -- Keir


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