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

Yifei Jiang

Hi John,
Yes, we believe that the new interrupt architecture and an IOMMU, which is regarded as a pass-through method as we knew, can indeed minimize the number of times of GuestOS exiting. However, this proposal targets to optimize user-level virtual devices that require to be implemented using the trap-and-emulated paradigm, such as rtc and UART. 
When accessing these devices, I/O path overheads caused by context switches between Guest OS and Host OS always dominate in the whole trap-and-emulated overheads.
Therefore, our proposal can improve the performance by kernel-bypassing I/O paths. According to our experiments performed on the RISC-V QEMU emulator and KVM hypervisor, the performance of UART implemented using our solution becomes 2X faster than the one in the original system. 

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