Re: [Xen-devel] Really need to pin page tables?

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Samuel Thibault wrote:
> Hello,
>
> In extras/mini-os/arch/x86/mm.c:new_pt_frame, Mini-OS pins its L1,
> L2, and L3 page tables.  Does that really make a difference from the
> Hypervisor point of view?  I mean, once L4 is pinned, pointing to these,
> and thus their content has been checked, is there any performance
> difference?
>   
Shouldn't be.  Pinning an L4 implicitly pins everything else below it.  
The only reason to pin the leafy parts of a pagetable is if you want to 
play games with incrementally pinning the pagetable, or if you want to 
pull them apart and rearrange the pieces for some reason.  For example, 
I do incremental pagetable pins in the Xen/pvops kernel to limit the 
number of pte locks I need to hold at any one time.
   J

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