yahoo-eng-team team mailing list archive

[Kashyap Chamarthy <kchamart@xxxxxxxxxx>]

Public bug reported:

In addition to the existing 'virt-ssbd', future AMD CPUs will have _two_ 
ways to deal with SSBD (Speculative Store Bypass Disable).  To that AMD
will be introducing two more[1][2] CPU flags:

    amd-ssbd
    amdb-no-ssb

It is recommended to add the above two flags to the whitelist of Nova's
`cpu_model_extra_flags` config attribute -- for stable branches (Queens,
Pike and Ocata).

For Rocky and above release, no such white-listing is required, since we
allow free-form CPU flags[3].

    * * *

Additional notes (from the QEMU mailing list thread[4]) related to
performance and live migration:

  - tl;dr: On an AMD Compute node, a guest should be presented with 
    'amd-ssbd', if available, in preference to 'virt-ssbd'.

    Details: Tom Lendacky from AMD writes[4] -- "The idea behind
    'virt-ssbd' was to provide an architectural method for a guest to do
    SSBD when 'amd-ssbd' isn't present.  The 'amd-ssbd' feature will use
    SPEC_CTRL which is intended to not be intercepted and will be fast.
    The use of 'virt-ssbd' will always be intercepted and therefore will
    not be as fast.  So a guest should be presented with 'amd-ssbd', if
    available, in preference to 'virt-ssbd'."

  - It safe to use 'amd-ssbd' (it is an architectural method for a guest
    to do SSBD) in a guest which can be live migrated between different
    generations/families of AMD CPU.


[1] libvirt patch:
    https://www.redhat.com/archives/libvir-list/2018-June/msg01111.html 
[2] QEMU patch:
    https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg00222.html
[3] http://git.openstack.org/cgit/openstack/nova/commit/?id=cc27a20 --
    libvirt: Lift the restriction of choices for `cpu_model_extra_flags`
[4] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg02301.html

** Affects: nova
     Importance: Undecided
         Status: New


** Tags: pike-backport-potential queens-backport-potential security

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https://bugs.launchpad.net/bugs/1777460

Title:
  Whitelist two more SSBD-related CPU flags for AMD ('amd-ssbd',  'amd-
  no-ssb')

Status in OpenStack Compute (nova):
  New

Bug description:
  In addition to the existing 'virt-ssbd', future AMD CPUs will have _two_ 
  ways to deal with SSBD (Speculative Store Bypass Disable).  To that AMD
  will be introducing two more[1][2] CPU flags:

      amd-ssbd
      amdb-no-ssb

  It is recommended to add the above two flags to the whitelist of Nova's
  `cpu_model_extra_flags` config attribute -- for stable branches (Queens,
  Pike and Ocata).

  For Rocky and above release, no such white-listing is required, since we
  allow free-form CPU flags[3].

      * * *

  Additional notes (from the QEMU mailing list thread[4]) related to
  performance and live migration:

    - tl;dr: On an AMD Compute node, a guest should be presented with 
      'amd-ssbd', if available, in preference to 'virt-ssbd'.

      Details: Tom Lendacky from AMD writes[4] -- "The idea behind
      'virt-ssbd' was to provide an architectural method for a guest to do
      SSBD when 'amd-ssbd' isn't present.  The 'amd-ssbd' feature will use
      SPEC_CTRL which is intended to not be intercepted and will be fast.
      The use of 'virt-ssbd' will always be intercepted and therefore will
      not be as fast.  So a guest should be presented with 'amd-ssbd', if
      available, in preference to 'virt-ssbd'."

    - It safe to use 'amd-ssbd' (it is an architectural method for a guest
      to do SSBD) in a guest which can be live migrated between different
      generations/families of AMD CPU.

  
  [1] libvirt patch:
      https://www.redhat.com/archives/libvir-list/2018-June/msg01111.html 
  [2] QEMU patch:
      https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg00222.html
  [3] http://git.openstack.org/cgit/openstack/nova/commit/?id=cc27a20 --
      libvirt: Lift the restriction of choices for `cpu_model_extra_flags`
  [4] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg02301.html

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