pmap: On a failed fcmpset don't repeat tests that won't change

[alc]

On arm64, the new pmap code is smaller by 60 bytes.

[andrew]

Is it possible for the hardware to change the AF or DBM flags between loading the pte and the fcmpset in the arm64 pmap?

[alc]

In D31014#697898, @andrew wrote:

Is it possible for the hardware to change the AF or DBM flags between loading the pte and the fcmpset in the arm64 pmap?

The hardware can change the AF bit from 0 to 1. The hardware does not change the DBM bit, only the OS does. If the DBM bit is 1, then the hardware can change ATTR_S1_AP(ATTR_S1_AP_RO) to ATTR_S1_AP(ATTR_S1_AP_RW) when the mapping is first written through, which amounts to changing a bit from 1 to 0.