+ *
+ * We used to clear some PSR bits here but that requires slow
+ * serialization. Fortuntely, that isn't really necessary.
+ * The rationale is as follows: we used to clear bits
+ * ~PSR_PRESERVED_BITS in PSR.L. Since
+ * PSR_PRESERVED_BITS==PSR.{UP,MFL,MFH,PK,DT,PP,SP,RT,IC}, we
+ * ended up clearing PSR.{BE,AC,I,DFL,DFH,DI,DB,SI,TB}.
+ * However,
+ *
+ * PSR.BE : already is turned off in __kernel_syscall_via_epc()
+ * PSR.AC : don't care (kernel normally turns PSR.AC on)
+ * PSR.I : already turned off by the time fsys_bubble_down gets
+ * invoked
+ * PSR.DFL: always 0 (kernel never turns it on)
+ * PSR.DFH: don't care --- kernel never touches f32-f127 on its own
+ * initiative
+ * PSR.DI : always 0 (kernel never turns it on)
+ * PSR.SI : always 0 (kernel never turns it on)
+ * PSR.DB : don't care --- kernel never enables kernel-level
+ * breakpoints
+ * PSR.TB : must be 0 already; if it wasn't zero on entry to
+ * __kernel_syscall_via_epc, the branch to fsys_bubble_down
+ * will trigger a taken branch; the taken-trap-handler then
+ * converts the syscall into a break-based system-call.