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We did the experiment based on our own settings and not yet consider the SBI extension proposal.
Please consider the approach in #278 with one additional condition: Any platform that supports configurations more than M-S-U should not provide a PMU with "bypass-sbi" attribute, like QEMU virt. Neither VS- nor HS- usage will be affected by this bit. Then, you ask, what about emulating a platform that aims to only runs on M-S-U machines? Well, the one who ports the platform to QEMU/other simulators should put some warning message when the attempt to write happens, rather than implement the whole save-restore just for the PMU status.
We don't need to add many CSRs. Just one bit in hpmevent*.