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[20090215] Xen news: Dom0 with PAE, PCI passthrough, migration
I have outlined the need for PAE with Xen in a previous posting. In the recent past, PAE has become the default in Linux Xen kernels, and as a result, running Linux on a NetBSD-Dom0 was not possible due to lack of PAE in Dom0. This has changed now, Manuel Bouyer has added PAE support for NetBSD/Xen's Dom0 support with a XEN3PAE_DOM0 kernel config now. Manuel's posting to port-xen outlines the PAE work for Dom0.

Other work that's underway is PCI passthrought: This feature that was available with Xen2 on NetBSD was changed in Xen3, and kernel support for DomUs to access specific devices on a PCI bus is underway. Currently, NetBSD DomUs need a Linux Dom0 to pass access to the PCI bus, but this is expected to change soon. First steps to pass single PCI devices from a NetBSD Dom0 to a DomU are made, and we can look forward for more of this to come. Again, see posting to port-xen for more details.

The last item is just a quick heads-up in reply to Manuel's posting: Jean-Yges Migeon is currently working to get saving and restoring of DomUs as well as the associated migration of domains between several Dom0s -- i.e. to move a running DomU from one machine to another -- working.

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[20080124] NetBSD Xen/i386 got PAE support (and why this is good)
A 32bit CPU can normally address 2^32 bytes, which is about 4GB RAM. As that's not enough for everyone any more today, one possible solution is to move to more address bits, which is what 64bit CPUs are for. Another alternative is to add some fancy magic to do paging/banking, which 16-bit Intel CPUs used "back then" when Protected Mode was still a topic. Today, address space beyond 4GB RAM can be addressed using the Physical Address Extension (PAE) on Intel (and compatible) CPUs.

So far, NetBSD never got PAE-support, as its amd64-port worked early and very well, so people with demand for more than 4GB ram usually moved to an appropriate hardware platform, and called it good. One case where PAE support was an issue is Xen, though: Linux usually has PAE enabled these days with Xen3 Dom0s, and DomUs running on a PAE-enabled Dom0 also have to support PAE. As a consequence, it was not easily possible to use NetBSD DomUs with contemporary Linux/Xen distributions.

Thanks to long-term NetBSD/Xen-hacker Manuel Bouyer, this has changed now: Manuel has added support for PAE to the NetBSD/Xen port, and he has merged the corresponding CVS branch to NetBSD-current now. There are separate kernels, XEN3PAE_DOMU and INSTALL_XEN3PAE_DOMU, which have PAE-support enabled, and which are expected to show up in daily builds soonish.

While this work does not add PAE support for NetBSD/i386 in general, it brings it a big step forward, and it may just be a matter of time until NetBSD/i386 also gets PAE support.

Implementation details can be found in src/sys/arch/i386/include/pte.h and src/sys/arch/i386/include/pmap.h, btw -- see Manuel's mails to port-xen for more information.

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