On Tue, Aug 29, 2017 at 11:40:30AM +1000, David Gibson wrote: > On Mon, Aug 28, 2017 at 10:53:56AM -0700, Ram Pai wrote: > > On Thu, Aug 24, 2017 at 12:54:48PM +1000, Paul Mackerras wrote: > > > > > > We could either have two u16 fields for the number of keys for data > > > and instruction, or we could have a u32 field for the number of keys > > > and a separate bit in the flags field to indicate that instruction > > > keys are supported. Which would be preferable? > > > > the second choice is more confusion-proof; to me atleast. > > > > The first choice gives a illusion that there are 'x' number of data keys > > and 'y' number of instruction keys; which is not exactly true. > > Ah.. can you elaborate?
On power8 and power9, there are only 32 keys, each key can be configured to disable data-access and instruction-access. The first choice, will report 32 keys for data-access and 32 keys for instruction-access. To a casual on-looker it gives an impresssion that there are 32 keys for data-access and 32 keys for instruction-access; 64 keys in total. And that is what I think can be the cause for confusion. RP