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

To: Anthony Liguori <aliguori@xxxxxxxxxx>
Subject: Re: [Xen-devel] Essay on an important Xen decision (long)
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 11 Jan 2006 10:08:38 +0000
Cc: "Magenheimer, Dan \(HP Labs Fort Collins\)" <dan.magenheimer@xxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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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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