CPU utilization is a known issue with UHCI emulation. I spent a short time poking around the code and USB specifications trying to come up with a way to detect "idle" periods where we don't need to poll the frame list at 1000 Hz.
I was hoping to find a solution to detect an "idle" UHCI state, i.e. a stable state where the guest is waiting for UHCI to report events and the guest isn't issuing new transfers. If the idle state can be detected, then UHCI stops its frame timer and protects the frame list and other control structure guest memory pages. When the guest writes to those memory pages again in order to issue a new USB transaction, we catch the write. UHCI unprotects the guest memory pages and turns the frame timer back on. Unfortunately the UHCI frame list, transfer descriptor, and QH structure is complicated. I didn't figure out whether it is feasible to protect all the necessary guest memory pages. Also, I'm not sure how easy it is for QEMU to protect guest memory in this fashion (for TCG and KVM). It seems like the VGA dirty memory stuff polls - it accumulates dirty memory bits and gets checked by the VGA emulation code in the necessary places - rather than intercepts writes. Any thoughts on this approach? Stefan
