add non-normative warning about perf implications if CTR is implemented without State Enable (riscv#1886)

Author: bcstrongx

src/smctr.adoc

@@ -425,6 +425,12 @@ When `hstateen0`.CTR=0, qualified control transfers executed while V=1 will cont
 _See <<indirect-csr>> for how bit 60 in `mstateen0` and `hstateen0` can also restrict access to `sireg*`/`siselect` and `vsireg*`/`vsiselect` from privilege modes less privileged than M-mode._
 ====
 
+[NOTE]
+[%unbreakable]
+====
+_Implementations that support Smctr/Ssctr but not Smstateen/Ssstateen may observe reduced performance.  Because Smctr/Ssctr introduces a significant number of new CSRs, it is desirable to avoid save/restore of CTR state when possible.  A hypervisor is likely to leverage State Enable to trap on the initial guest access to CTR state, delegating CTR and enabling save/restore of guest CTR state only once the guest has begun to use it.  Without Smstateen/Ssstateen, a hypervisor is required to save/restore guest CTR state on every context switch._
+====
+
 === Behavior
 
 CTR records qualified control transfers.  Control transfers are qualified if they meet the following criteria: