Re: Proposal: Delegating Exceptions from VS-mode or VU-mode to U-mode

Andrew Waterman

The N extension is effectively deprecated. We don’t see sufficient demand for user-level interrupts in managed/Unix-like environments to pursue that approach at this time.

We do see demand for this feature in embedded systems with two levels of privilege. In this case, the proposal is to run the trusted piece in M-mode and the less-trusted piece in “bare S-mode”: i.e., S-mode without virtual memory.

This approach covers the interesting use cases without additional architectural complication, and in particular, the hypervisor architecture need not care.

On Fri, Sep 18, 2020 at 1:03 AM Yifei Jiang via <> wrote:

Hi all,


In this proposal, we extended N extension and applied it to H extension for improving the performance of virtual I/O devices in the virtualization scenario. We proposed a new mechanism to delegate exceptions from VS-mode/VU-mode

to U-mode. A solution is also implemented based on QEMU simulator and KVM virtualization architecture. Evaluation results show that our proposal achieves nearly 2x faster synchronous I/O processing speed than the orginal system.


The attachment is the detailed proposal. Any comments are welcome.




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