Merge pull request #38 from riscv/frz-clarification

address issue #37, adding clarification of LCOFIFRZ behavior when no LCOFI trap is raised

body.adoc

@@ -731,14 +731,9 @@ When `sctrctl`.LCOFIFRZ=1 and a local counter overflow interrupt
 [NOTE]
 [%unbreakable]
 ====
-_Freeze on LCOFI ensures that the execution path leading to the sampled
-instruction (xepc) is preserved, and that the local counter overflow
-interrupt (LCOFI) and associated Interrupt Service Routine (ISR) do not
-displace any recorded transfer history state. It is the responsibility
-of the ISR to clear FROZEN before xRET, if continued control transfer
-recording is desired._
-
-_LCOFI refers only to architectural traps directly caused by a local counter overflow. If a local counter overflow interrupt is recognized without a trap, for instance by reading mip, FROZEN is not automatically set._
+_Freeze on LCOFI ensures that the execution path leading to the sampled instruction (`xepc`) is preserved, and that the local counter overflow interrupt (LCOFI) and associated Interrupt Service Routine (ISR) do not displace any recorded transfer history state. It is the responsibility of the ISR to clear FROZEN before xRET, if continued control transfer recording is desired._
+
+_LCOFI refers only to architectural traps directly caused by a local counter overflow. If a local counter overflow interrupt is recognized without a trap, FROZEN is not automatically set.  For instance, no freeze occurs if the LCOFI is pended while interrupts are masked, and software recognizes the LCOFI (perhaps by reading `stopi` or `sip`) and clears `sip`.LCOFIP before the trap is raised.  As a result, some or all CTR history may be overwritten while handling the LCOFI.  Such cases are expected to be very rare; for most usages (e.g., application profiling) privilege mode filtering is sufficient to ensure that CTR updates are inhibited while interrupts are handled in a more privileged mode._
 ====
 Similarly, on a breakpoint exception that traps to M-mode or S-mode with `sctrctl`.BPFRZ=1, FROZEN is set by hardware. The breakpoint exception itself is not recorded.