Supporting consistency of vcpu_runstate_info across cpus

[Juergen Gross <jgross@suse.com>]

A guest kernel can use the vcpu_op hypercall sub-op
VCPUOP_register_runstate_memory_area to get a copy of the
vcpu_runstate_info of a vcpu mapped into its memory. As this structure
has no update indicator it is only save to be read by the vcpu it is
containing the runstate information of.

Being able to read the runstate info of another cpu is required e.g.
by the Linux kernel to be able to calculate vruntime: see

http://lists.xen.org/archives/html/xen-devel/2016-05/msg01790.html

I'd suggest to add an "update in progress" indicator in the highest
bit of vcpu_runstate_info->state_entry_time as this structure element is
already used to detect vcpu scheduling when vcpu_runstate_info is read
by the owning vcpu.

The question is how to enable setting this indicator, as the guest must
be able to cope with it (I believe the Linux kernel would just run fine,
but we can't be sure this is true for all guests).

I see the following possible solutions:

a) Introduce a new vcpu_op hypercall sub-op for mapping the
   vcpu_runstate_info with update indicator support (a guest supporting
   this would try the new sub-op first and could fall back to
   VCPUOP_register_runstate_memory_area in case of ENOSYS).

b) Add a virtual MSR to switch on the feature (not being able to set the
   appropriate bit would indicate the feature not being available). This
   is the variant KVM is using. Does ARM have something like MSRs?

c) Add another hypercall to switch on the feature (similar to
   XENVER_get_features we could have a XENVER_set_features).

Any preferences?


Juergen

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