stable/11 r329462 - Meltdown/Spectre MFC questions

Jeremy Chadwick jdc at
Sat Feb 17 19:47:29 UTC 2018


Do the following new loader tunables and sysctls have documentation
anywhere?  I ask because I wish to know how to turn all of this off (yes
you heard me correctly), as not all systems necessarily require
mitigation of these flaws.

Best I can tell from skimming source:

  - Description: Page Table Isolation enabled
  - Loader tunable, visible in sysctl (read-only)
  - Integer
  - Default value: depends on CPU model and capabilities, see
    function pti_get_default(); looks like AMD = 0, any CPU with
    RDCL_NO capability enabled = 0, else 1

  - Description: Indirect Branch Restricted Speculation active
  - sysctl (read-only)
  - Integer
  - Real-time indicator as to if IBRS is currently on or off

  - Description: Disable Indirect Branch Restricted Speculation
  - Loader tunable and sysctl tunable (read-write)
  - Integer
  - Default value: unsure.  Variable declaration has 1 but
    SYSCTL_PROC() macro has 0.

Thank you.

| Jeremy Chadwick                                   jdc at |
| UNIX Systems Administrator       |
| Making life hard for others since 1977.             PGP 4BD6C0CB |

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