Re: Disabling and re-enabling extensions
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The hypervisor spec specifically says that implementations should have writable misa so this isn't theoretical:
"The hypervisor extension is enabled by setting bit 7 in the misa CSR, which corresponds to the letter H. When misa is clear, the hart behaves as though this extension were not implemented, and attempts to use hypervisor CSRs or instructions raise an illegal instruction exception. Implementations that include the hypervisor extension are encouraged not to hardwire misa, so that the extension may be disabled. "
And, of course, the misa register says that extensions may be disabled, that both E and I may be implemented and that misa is the way to select between them, etc. I don't know if anybody has actually done those things, though.
On Thu, Sep 10, 2020 at 4:05 PM Allen Baum <allen.baum@...> wrote: