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[Xen-devel] Discovering L0 hypervisor features from L2

Hi all

One of the things required to make a Xen system run well on Hyper-V is
to make Dom0 use as many paravirtualised interfaces as possible.

For starters, we would really want Dom0 Linux kernel to use Hyper-V's PV
drivers. But the core VMBus driver is gated by cpuid leaves. If cpuid
returns Xen's signature, VMBus driver won't be loaded.

We will want to different method other than cpuid leaves to communicate
to Linux "this Xen hypervisor is running on top of Hyper-V, thus it is
okay to use Hyper-V drivers". We may also want to be able to indicate
what features are available directly from L0.

There are several ways of doing this:

1. Use a hypervisor specific cpuid leaf.
2. Use an MSR reserved for software use.
3. Use a hypercall.

I generally prefer going with either 1 or 2.

Last but not least, we should make the discovery mechanism generic for
any L0 hypervisor, such that we can easily add support for running Xen
in L0 in the future.

Thoughts, comments and preferences?


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