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Re: [Xen-devel] Two shadow page tables for HVM

Emre Can Sezer wrote:
Wouldn't this mean that the two page tables are NOT synchronized?  When
we switch paging modes, wouldn't we have to rebuild the entire shadow
page tables from guest?

No. When updating shadows from guest, the shadow code will update the
changes for each existing shadow of the page.

When considering the performance penalties of flushing the kernel page
tables from the TLB, how significant is traversing all the shadow page
tables for the guest kernel and updating their permissions?  If there
isn't an order of magnitude of difference, it might be reasonable to
take the short cut in implementation.

It's up to the permission you're updating and how widely you want these permissions applied. If (e.g. NX bit) one bit in the upper level set permission for the whole part of the tree it maps, then you can just change permissions on the top level shadows. Be sure, though, to cope with the fault you get correctly.


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