Re: [Xen-users] Re: [Xen-devel] VMX Hardware

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Aug 2005, at 19:09, Anthony Liguori wrote:

> The Pacifica spec does mention optional hardware support for "Nested 
> Page Tables" which is essentially hardware shadow page tables.
> Should be interesting to see the performance difference between 
> software/hardware shadow page tables.
I have doubts about the performance of hardware page tables, unless 
there's a clever TLB design. A pagetable walk which is four memory 
accesses on normal x86/64 becomes twenty accesses in nested mode!
It may be that shadow pagetables, by providing a bigger 'cache' for 
guest->host mappings, can actually perform better on many workloads.
I guess we'll see -- if silicon is produced that supports that mode, 
someone is bound to add support to Xen.
 -- Keir


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