ot_otp_engine: Replace BHs with short timers#310
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jwnrt
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Not an expert of this part of the code but the change looks reasonable.
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Being OT-offboarded tonight, keeping the review as comment-only :-)
From testing, it seems that the use of Bottom Halves on hosts under higher processing loads (where QEMU is more often pre-empted) can produce inconsistent / slow results. Timers with a short timeout provide the same decoupling functionality as the BH but with more consistency and expedience. This could be especially problematic for SW which expects digest writes to be processed within a certain time. If handling of the BH was deferred long enough, then enough guest code could be processed for it to look like e.g. 10 ms had passed, exceeding common SW timeouts. This could be seen happening occasionally under high processing loads. Signed-off-by: Alex Jones <alex.jones@lowrisc.org>
From testing, it seems that the use of Bottom Halves on hosts under higher processing loads (where QEMU is more often pre-empted) can produce inconsistent / slow results. Timers with a short timeout provide the same decoupling functionality as the BH but with more consistency and expedience. Signed-off-by: Alex Jones <alex.jones@lowrisc.org>
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See also some relevant discussion in this comment: #178 (comment). In general without a multithreaded implementation we currently prefer using a short (~100ns or so) timer over Bottom Halves for decoupling larger processing tasks and yielding back to the vCPU for a short while.
From personal testing of more involved OpenTitan integration flows, I found that when running under heavy processing loads (running many tests in parallel, such that QEMU is being pre-empted quite often), these BHs could occasionally be quite inconsistent and take a surprisingly long time to be scheduled. This was contributing to unexpected test failures due to timing - while timing is not expected to be accurate, the BH was inconsistently causing an order of magnitude difference than what is expected for some OTP operations.