[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Xen-users] Re: Xen is a feature

Ingo Molnar wrote:
There is in fact a way to get dom0 support with nearly no changes to Linux, but it involves massive changes to Xen itself and requires hardware support: run dom0 as a fully virtualized guest, and assign it all the resources dom0 can access. It's probably a massive effort though.

I've considered it for kvm when faced with the "I want a thin hypervisor" question: compile the hypervisor kernel with PCI support but nothing else (no CONFIG_BLOCK or CONFIG_NET, no device drivers), load userspace from initramfs, and assign host devices to one or more privileged guests. You could probably run the host with a heavily stripped configuration, and enjoy the slimness while every interrupt invokes the scheduler, a context switch, and maybe an IPI for good measure.

This would be an acceptable model i suspect, if someone wants a 'slim hypervisor'.

We can context switch way faster than we handle IRQs. Plus in a slimmed-down config we could intentionally slim down aspects of the scheduler as well, if it ever became a measurable performance issue. The hypervisor would run a minimal user-space and most of the context-switching overhead relates to having a full-fledged user-space with rich requirements. So there's no real conceptual friction between a 'lean and mean' hypervisor and a full-featured native kernel.

The context switch would be taken by the Xen scheduler, not the Linux scheduler. It's how interrupts work under Xen: an interrupt is taken, Xen schedules the domain that owns the interrupts (dom0 usually), which then handles the interrupt. The Linux scheduler would only be involved if you thread your interrupt handlers.

This context switch is necessary regardless of how dom0 is integrated into Linux; it's simply a side effect of implementing device drivers outside the kernel (in this context, the kernel is Xen, and dom0 is just another userspace, albeit with elevated privileges. The Linux equivalent to dom0 is a process that uses uio.

Do not meddle in the internals of kernels, for they are subtle and quick to 

Xen-users mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.