Re: [RFC] Correct/fast timestamping in apps under Xen [0 of 4]

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 10/02/09 10:50, Dan Magenheimer wrote:
> The premises are:
>
> 1) A large and growing percentage of servers running
>    Xen have a "reliable" TSC and Xen can determine
>    conclusively whether a server does or does not
>    have a reliable TSC.
> 2) A small but growing percentage of servers running
>    Xen implement the rdtscp instruction but Xen does
>    not and will not expose this instruction to guest
>    OSes.
> 3) Xen is able to track the "incarnation" number for
>    a guest.  This number will increase whenever a
>    guest is restored or migrated (and possibly more
>    frequently).  Optionally, an administrator can
>    explicitly mark a guest as "landlocked", disallowing
>    save/restore/migration for that guest.
> 4) Apps can become "virtualization aware" in that
>    they can access certain information directly
>    from Xen utilizing an OS-independent mechanism.
>    This information includes not only "Am I running
>    on Xen?" but also, for example, "Is TSC reliable
>    on this physical machine?", "Is rdtsc emulated
>    or native on this virtual machine?",  "What is
>    the current incarnation number for this virtual
>    machine?", "Is this virtual machine landlocked?",
>    "What are the pvclock parameters for this
>    virtual machine?", etc.
>   

I think you're missing a couple of premises here:

    0) It is not possible to update the existing APIs and ABIs available
    to applications to meet the most demanding performance requirements.

    5) There will be enough important applications (ie, broadly used,
    rather than a few in-house apps) whose developers are willing to
    update them to your new proposed ABI to justify adding and
    maintaining these new ABIs.

Without discussing these, you're presupposing your solution is necessary.

    J