IBM AIX Continuous Availability Features - IBM Redbooks

For any load/store, the CPU retrieves the memory key assigned to the targeted page during
the translation process. The key number is used to select the bit pair in the AMR that defines
if an access is permitted. A data storage interrupt results when this check fails.
The AMR is a per-context register that can be updated efficiently. The TLB/ERAT contains
storage key values for each virtual page. This allows AMR updates to be efficient, because
they do not require TLB/ERAT invalidation. The POWER hardware enables a mechanism
that software can use to efficiently change storage accessibility.
Ideally, each storage key would correspond to a hardware key. However, due to the limited
number of hardware keys with current Power Architecture, more than one kernel key is
frequently mapped to a given hardware key. This key mapping or level of indirection may
change in the future as architecture supports more hardware keys.
Another advantage that indirection provides is that key assignments can be changed on a
system to provide an exclusive software-to-hardware mapping for a select kernel key. This is
an important feature for testing fine granularity keys. It could also be used as an SFDC tool.
Kernel keys provide a formalized and abstracted API to map kernel memory classes to a
limited number of hardware storage keys.
For each kernel component, data object (virtual pages) accessibility is determined. Then
component entry points and exit points may be wrapped with “protection gates”. Protection
gates change the AMR as components are entered and exited, thus controlling what storage
can be accessed by a component.
At configuration time, the module initializes its kernel keyset to contain the keys required for
its runtime execution. The kernel keyset is then converted to a hardware keyset. When the
module is entered, the protection gates set the AMR to the required access authority
efficiently by using a hardware keyset computed at configuration time.
User keys work in application programs. The keys are a virtualization of the PowerPC storage
hardware keys. Access rights can be added and removed from a user space AMR, and an
user key can be assigned as appropriate to an application’s memory pages. Management
and abstraction of user keys is left to application developers.
3.7.2 System management support for storage keys
You can use smitty skeyctl to disable (keys are enabled by default) Kernel keys, as shown
in Example 3-18.
Note: You may want to disable kernel keys if one of your kernel extensions is causing key
protection errors but you need to be able to run the system even though the error has not
been fixed.
Example 3-18 Storage protection keys SMIT menu
Storage Protection Keys
Move cursor to desired item and press Enter.
Change/Show Kernel Storage Protection Keys State
F1=Help F2=Refresh F3=Cancel F8=Image
F9=Shell F10=Exit Enter=Do
Chapter 3. AIX advanced continuous availability tools and features 79