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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 10 Jan 2006, at 19:55, Anthony Liguori wrote:

> You seem to conclude that the only possible solutions are making the 
> dom0 either P==M or P2M.  Is it not possible to make dom0 VP?
> If the only issue for making dom0 VP is DMA, wouldn't it be easier to 
> modify the Linux DMA subsystem[1] to make a special hypercall to 
> essentially pin a VP to a particular MFN that could be used for the 
> DMA?  One could imagine the hypervisor reversing low memory 
> specifically for DMA such that bounce buffers could be avoided too.
> VP makes a lot of interesting memory optimizations considerably easier 
> (memory compacting, swapping, etc.).
On an architecture where VP is cheaper to implement than on x86, it may 
well make sense to do that in preference to P2M. As you say, it makes 
certain future extensions less of a pain to implement.
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


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