Design session "Using the new VCPU counters in XAPI"

[Matias Ezequiel Vara Larsen <matiasevara@xxxxxxxxx>]

Participants: Pau, Edwin, Matias

- the semantics of the new interface for querying registers is now async, this 
would require a initialization phase in xcp-rrdd before query the counters.
- why this 
https://github.com/xenserver/xen.pg/blob/XS-8.2.x/master/mixed-domain-runstates.patch
 is not up-streamed?
- we need to implement a protocol to ensure consistency between 
producer(hypervisor) and consumer(user-space)
- we can get inspiration from in 
xen/arch/x86/time.c:__update_vcpu_system_time(). This relies on 
version_update_begin() and version_update_end()
- get inspiration from the comment at public/xen.h in the struct 
vcpu_time_info{} in which a version number is used
- get inspiration from arch/x86/time.c:read_xen_timer()
- we need something in user-space that relies on the same mechanism, i.e., get 
a consistent value if version number is the same before and after reading the 
structure.
- add bindings for acquiere resource see 
https://github.com/xapi-project/xen-api/blob/master/ocaml/xenopsd/c_stubs/xenctrlext_stubs.c#L460)
- also get inspiration from 2529c85
- we cannot find where is the implementation of this protocol in user-space. 
Xenctrl should implement it somewhere