Re: proposal for stateen CSRs

Jonathan Behrens <behrensj@...>

The proposed mstateenX CSRs feels very similar to misa. It almost seems like you could achieve the same thing as this proposal just by adding sisa and hisa (plus misa2, misa3, ...), but I think I've convinced myself that wouldn't quite work.

I'll also point out that we'll probably want to reserve a bit in [m|s]stateen0 for the H-extension.


On Tue, Apr 20, 2021 at 11:01 PM John Hauser via <> wrote:
Allen Baum wrote:
> Questions: (I suspect I know the answers, but want to be clear):
>   -if  mstateenY[n] is cleared, and s-mode code tries to read/write a CSR
> that is controlled by that bit
>     -- does the access trap?
>     -- else does it cause read/writes of the CSR from Smode
>         --- to return zero/have no effect
>         --- to return the value it had at the time that the bit was
> cleared/have no effect
> Or put another way: what exactly does "control access" mean?

I forgot to say what that means, didn't I?  Good point!

To quote Admiral Ackbar:  "It's a trap!"  You get an illegal
instruction trap, unless you're executing in a virtual machine (V = 1)
and the access is _not_ being blocked by the mstateen CSRs, in which
case you get a virtual instruction trap.

(Related to that, it's on my to-do list to add to the Privileged
Architecture's hypervisor chapter a better general explanation for
when you should get a virtual instruction trap instead of an illegal
instruction trap.)

    - John Hauser

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