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

[Gianluca Guida <gianluca.guida@xxxxxxxxxxxxx>]

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.

Gianluca

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