Power ISA/Privilege States
At the moment very little information exists about the Ultravisor State. It is not mentioned in Power ISA version 2.07 documents at all, and version 3.0B only mentions it as a possible privilege of instructions. There is no official documentation of a UV - Ultravisor State Machine State Register bit, although some source code does reference its existence.
Skiboot documentation mentions this as one of "the four rings".
IBM has confirmed to Raptor in direct messaging that the ultravisor state does not exist in POWER9, despite some material continuing to reference it. This information was also made public on Twitter.
Hypervisor State is indicated by the HV (bit 3) of the Machine State Register, and is normally used by a hypervisor. An operating system running without a hypervisor can run in Hypervisor State, with its userland in Problem State and avoid using Privileged State altogether.
Hypervisor State was introduced in POWER4, although for some time it was not included in documentation, appearing only as a reserved bit in the Machine State Register.
Privileged State, also called Supervisor Mode, is normally used by an operating system running on top of a hypervisor.
Problem State, also called User Mode, is indicated by the PR (bit 49) of the Machine State Register.
|P||Yes||Yes||a privileged instruction.|
|O||Yes||Yes||an instruction that is treated as privileged or nonprivileged (or hypervisor, for mtspr), depend-
ing on the SPR or PMR number.
|PI||No||Yes||an instruction that is illegal in privileged state.|
|H||Yes||Yes||an instruction that can be executed only in hypervisor state|
|PH||Yes||No||a hypervisor privileged instruction if Category Embedded.Hypervisor is implemented; otherwise
denotes a privileged instruction.
|M||Yes||No||an instruction that is treated as privileged or nonprivileged, depending on the value of the UCLE
bit in the MSR
|U||No||Yes||an instruction that can be executed only in ultravisor state|
- P9 XIVE Exploitation > I - Device-tree updates "reg property contains the addresses & sizes for the register ranges corresponding respectively to the 4 rings: Ultravisor level, Hypervisor level, Guest OS level, User level"
- File:AFRL-RI-RS-TR-2017-021.pdf. IBM. 2017-02
- Lynn, Justin. tweet
- Kerr, Jeremy. OpenPOWER: building an open-source software stack from bare metal (video). Linux.conf.au 2015