Re: Smstateen for Zcmt


Greg Favor
 

On Mon, May 16, 2022 at 10:02 AM Tariq Kurd <tariq.kurd@...> wrote:
Thanks for the very helpful email - I think this is the first time that a new state enable has been added since Smstateen was ratified.
The reason that it's required is that there's a new CSR: JVT, which must not be used as a covert channel.
My intention was only to disable this CSR, and not to disable any of the new instructions from any of the Zc extensions.
The question which arose was prompted by this text from the Smstateen spec:

"In some cases, the bits of the stateen CSRs will have a dual purpose as enables for the ISA
extensions that introduce the controlled state."

And so the question is whether the instructions are also disabled.

The intent is that if Smstateen is needed to provide a backward compatibility mechanism, then the extension AS A WHOLE would be enabled or disabled.  (This, as I noted earlier, should be a rare case.)

In the common use case for Smstateen and the case of Zcmt, this does not apply and only access to the new JVT CSR would be enabled or disabled,

Greg

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