Can someone provide a definitive answer (Andrew?) as to the architectural intent of whether implementations supporting new architecture extensions must maintain backward compatibility with "legacy" M mode software (and User/Supervisor software running under that M-mode software) that is unaware of the extensions yet the extensions are left enabled?  (This becomes more relevant as standard M-mode reference boot software and commercial TEE software products become established in the RISC-V Linux world.)

'No' says that it is alright to presume or require use of non-legacy M-mode boot software (or modifications to that software) that will disable the relevant misa bits if necessary.  This hopefully is the answer.

'Yes' says that the implementation must reset further architectural state past what is defined in the Privileged spec so as to ensure well-behaved Supervisor code, and somewhat well-behaved User code, isn't affected by the unexpected yet enabled extensions. In the case of the Hypervisor extension, for example, three bits of CSR state must reset to specific values.  And future extensions must have this characteristic that there does exist a set of fixed reset values to accomplish this.  (If 'Yes', then it might be useful for the Hypervisor spec to specify what additional hart reset state is necessary to satisfy this architectural intent/requirement.)

Greg