Re: [PATCH 26/26] KVM: PPC: Add Documentation about PV interface

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Am 27.06.2010 um 10:34 schrieb Avi Kivity <avi@xxxxxxxxxx>:

On 06/26/2010 02:25 AM, Alexander Graf wrote:
We just introduced a new PV interface that screams for documentation. So here it is - a shiny new and awesome text file describing the internal works of
the PPC KVM paravirtual interface.


+Querying for existence
+======================
+
+To find out if we're running on KVM or not, we overlay the PVR register. Usually +the PVR register contains an id that identifies your CPU type. If, however, you +pass KVM_PVR_PARA in the register that you want the PVR result in, the register
+still contains KVM_PVR_PARA after the mfpvr call.
+
+    LOAD_REG_IMM(r5, KVM_PVR_PARA)
+    mfpvr    r5
+    [r5 still contains KVM_PVR_PARA]
+
+Once determined to run under a PV capable KVM, you can now use hypercalls as
+described below.


On x86 we allow host userspace to determine whether the guest sees the paravirt interface (and what features are exposed). This allows you to live migrate from a newer host to an older host, by not exposing the newer features.

A very good idea indeed. Let's postpone that to when we expose enough state to make live migration possible.

Alex

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