Messages in this thread | | | Subject | Re: arch/xen is a bad idea | Date | Thu, 16 Dec 2004 01:14:09 +0000 | From | Ian Pratt <> |
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> You merge xen hooks in mainline, but keep maintaining arch/xen > out-of-tree? You have to maintain it yourself, anyway, and having > hooks merged should make it easy.
Having the hooks merged would certainly help.
However, I think the main benefit of being in the mainline tree would be that it would make it easy for other developers to see how arch xen works and thus begin to take us into account when considering changes. If there's going to be a big effort to merge i386, x86_64 and xen going forward, it will certainly help if more people have looked through our code. We'll only get that kind of exposure to developers if we're in the mainstream tree.
The other key area is that our top priority is to decrease the number of files we need to modified from standard i386. For this to happen, we need to submit patches into i386 that abstract a few things behind macros/constants. For example, we'd like to abstract the test to see whether the CPU is in the kernel or not (we run the kernel in ring 1 rather than 0). If arch xen is in the tree, this kind of patch will make rather more sense to people.
> BTW if you merge xen as separate architecture, it will be *very* hard > to merge it back to i386. That patch would be huge, and would need to > go in "atomically".
I don't see it like that. While continuing to track changes in i386/x86_64, we'd restructure the code under arch xen such that it could build (or even boot) time switch between running native and over Xen. At some point the arch directory could then be renamed. This would be a big project, and one that would involve a lot more people than just the Xen team. Because the x86 architecture is so critical to Linux, I just can't see this happening unless the development takes place in the mainstream tree.
Of course, another reason for inclusion in mainstream is that there's a lot of momentum behind Xen in both the user and developer communities. My guess is that virtualization is going to be pretty pervasive in the data centre, and its one of the key technologies that Linux can win on. The paravirtualized approach used by Xen is going to offer better performance than full x86 virtualization, even if there's extra support in the processor.
Cheers, Ian
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