On 12/09/2015 08:45 AM, Michael Kerrisk (man-pages) wrote:

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* Explanation of what a protection domain is.

A protection domain is a unique view of memory and is represented by the
value in the PKRU register.

Out something about this in pkey(7), but explain what you mean by a
"unique view of memory".

Let's say there are only two protection keys: 0 and 1.  There are two
disable bits per protection key (Access and Write Disable), so a two-key
PKRU looks like:

|   PKEY0   |   PKEY1   |
| AD0 | WD0 | AD1 | WD1 |

In this example, there are 16 possible protection domains, one for each
possible combination of the 4 rights-disable bits.

"Changing a protection domain" would mean changing (setting or clearing)
the value of any of those 4 bits.  Each unique value of PKRU represents
a view of memory, or unique protection domain.