I want to bring back up the Yes/No question I posed a couple of weeks ago - in which I was looking for a clear architectural statement of principle as to whether:

A) A new architecture extension must maintain backward compatibility with unaware M, S, and U mode software while the extension is left enabled.  This tends to require additional resetting of key architectural state to achieve this - that (as Andrew agreed) should be specified in the extension's arch spec.  (In the case of the Hypervisor extension, for example, three bits of CSR state must be reset to specific values to provide backward compatibility for well-behaved M/S/U code.)

OR 

B) It is alright to presume or require use of extension-aware M-mode boot software that will disable the relevant misa bits as necessary (at which point there is no need for architectural specification of reset state to ensure backward compatibility, nor any need to worry about this).

Greg

On Mon, Jun 8, 2020 at 6:25 PM Andrew Waterman <andrew@...> wrote:

On Mon, Jun 8, 2020 at 6:18 PM Greg Favor <gfavor@...> wrote:

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.)

I don't think there's a clear statement of principle on the matter, so it is something for us to decide as a group.  In this particular case, if we can maintain compatibility with existing M-mode software by only resetting a few state bits, then I think we should reset a few state bits.

'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.)

Agreed, if we go this route, the hypervisor spec needs to clearly state which things need to be reset.