Comment 0 for bug 2019968

[Impact]

    Add the following feature bits for EPYC-Milan model and bump the version.
    vaes : Vector VAES(ENC|DEC), VAES(ENC|DEC)LAST instruction support
    vpclmulqdq : Vector VPCLMULQDQ instruction support
    stibp-always-on : Single Thread Indirect Branch Prediction Mode has enhanced
                      performance and may be left Always on
    amd-psfd : Predictive Store Forward Disable
    no-nested-data-bp : Processor ignores nested data breakpoints
    lfence-always-serializing : LFENCE instruction is always serializing
    null-sel-clr-base : Null Selector Clears Base. When this bit is
                                set, a null segment load clears the segment base

[Test Plan]

 * First of all we'll (and have in advance) run general regression tests

 * Qemu shall show to be aware of the new types
   # qemu-system-x86_64 -cpu ? | grep EPYC-Milan
    x86 EPYC-Milan (alias configured by machine type)
    x86 EPYC-Milan-v1 AMD EPYC-Milan Processor
    x86 EPYC-Milan-v2 AMD EPYC-Milan Processor

[Where problems could occur]

 * There are two areas to look at
   a) compat behavior on old systems - e.g. libvirt would now detect IBRS
      on such AMD chips and one might wonder about the change.
      E.g. compatibility would exist between old-code/new-code/old->new
      code; but any action (e.g. suspend resume) from new to old code
      might run into trouble (not supported that way but worth to mention
      for awareness)
   b) Migrations between systems - this should be covered by chip
      versioning but still is worth to mention. Versioning will recognize
      a formerly started system as v1 and continue to handle it that way.
      Only new started guests would become v2 and behave the new and
      improved way.

[Other Info]

 * n/a

---

https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg02082.html

https://github.com/qemu/qemu/commit/27f03be6f59d04bd5673ba1e1628b2b490f9a9ff.patch

This patch depends on the definitions that were added as part of the EPYC-Milan patch:

amd-psfd, stibp-always-on:
* https://github.com/qemu/qemu/commit/bb039a230e6a7920d71d21fa9afee2653a678c48.patch
Add feature bits for CPUID_Fn80000021_EAX:
* https://github.com/qemu/qemu/commit/b70eec312b185197d639bff689007727e596afd1.patch