Re: [Xen-devel] Essay on an important Xen decision (long)

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Mark Williamson wrote:

> > I imagine that you would have to always have shadow paging enable but
> > you could still do bulk updates ala writable page tables so the
> > performance cost should be minimal I would think.
> >
> > Trying to understand the memory system in more details so any additional
> > info is much appreciate :-)
> >    
> I don't see why that couldn't perform decently, although it'd have more 
> overhead than allowing the guest to manage its pagetables directly...  I 
> *thought* this was intended to be supported at some point, but I'm not sure 
> if it's been needed yet.  Others may have more concrete numbers for the 
> performance - I think writable PTs got benchmarked against shadowing at some 
> point.
>  
Just to be thorough, was the shadow paging code a "pure" shadow page 
table where ever PTE write trapped to the hypervisor or were bulk PMD 
updates sent to the hypervisor?
I'm surprised there would be a measurable difference with shadow paging 
as it should only require a potential allocation (which could be fast 
pathed) and in the normal case, a couple extra reads/writes.  I would 
think that cost would be overshadowed by the original cost of the 
context switch.
Of course, I guess it wouldn't be that much of a shock to me that the 
overhead is at least measurable...
Regards,

Anthony Liguori

> Cheers,
> Mark
>
>  
>
> > Thanks,
> >
> > Anthony Liguori
> >
> >    
> >
> > > If ia64 does decide to back off from the P==M route then I suspect VP
> > > is the way to go (which is I think how ia64 domU's currently work
> > > anyway).
> > >
> > > -- Keir
> > >      
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >    
>  

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