Re: [Xen-devel] [RFC] New shadow paging code

["Mike D. Day" <ncmike@xxxxxxxxxx>]

Tim Deegan wrote:


> Our code must deal differently with all the different combinations of
> shadow modes.  However, we expect that once a guest reaches its target
> paging mode, it will stay in that mode for a long time; and the host will
> never change its paging mode.  Rather than having a whole string of ifs in
> the code based on the current guest and host paging modes, we compile
> different code to deal with each pair of modes (2-on-2, 2-on-3, 2-on-4,
> 3-on-3, 3-on-4, 4-on-4).  (Direct mode is implemented as a special case of
> m-on-m, where m is the host's current paging level.)  While increasing the
> size of the hypervisor overall, this should greatly decrease both the cache
> footprint of the shadow code and reduce pipeline flushes from mispredicted
> branches.
Can you explain how this works? It is not obvious to me from the patch. 
In shadow2.c there is in fact a whole string of #ifs. What mechanism 
causes the file to be multiply compiled? Or do I misunderstand how this 
works?
Also, can you comment on the differences and similarities with the 
shadow code contributed by Ben Thomas?
thanks,

Mike Day

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