Re: proposal for stateen CSRs


John Hauser
 

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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