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    SubjectRe: Use CPUID to communicate with the hypervisor.
    Alok Kataria wrote:
    > Hi Gerd,
    > I really fail to see your point here. Maybe you can point out what am i
    > missing.
    > Think about the current situation, whenever there is migration to such a
    > tsc-is-different system , how does the guest come to know about the
    > frequency change, either through a $event or if it reboots it runs the
    > calibration algorithm.

    Well, that should be clearly defined, that is my point. When asking the
    hypervisor for the tsc instead of running a calibration loop, then we
    have a small bit of paravirtualization: The guest is aware that it runs
    on a hypervisor and just asks it directly. So while we are at it we can
    also define a way to communicate tsc freq changes between host and
    guest, so the cost of trap'n'emulate tsc reads can be avoided. Or we
    define "tsc is constant" and leave it to the hypervisor to make sure it
    actually appears being constant to the guest, even in case it changes on
    the host. But it must be defined one way or another, so the guest knows
    whenever it should expect the tsc frequency change or not. And in case
    we allow tsc changes, we also need a way to signal that to the guest.

    Is the tsc cpu leaf interface set in stone already (aka implemented in
    vmware versions released to public)?

    > What special things does Xen do at migration, which would be affected by
    > this interface ?

    paravirtualized xen guests have a paravirtual clock. That is a struct
    containing three pieces of information: system time, tsc counter for the
    last system time update, tsc frequency. The guest gets the current time
    by reading the system time and adding a delta calculated from current
    tsc, tsc of last systime update and tsc frequency. Handling tsc
    frequency changes is obviously trivial here, just update the field on
    the next systime update ;)


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