I'm getting 2x "0.00 slack (VIOLATED)" on hold time (min) during spef extraction STA. That's the only violation and I keep getting this despite loosening the parameters. What does "0.00" violation mean?

what is your clock period?

"CLOCK_PERIOD" : 20, "FP_CORE_UTIL" : 40

@User Hold Slack 0.00 (VIOLATED) means Timing is exactly met the Hold specification. You can ignore this Warning. Clock period does not have impact on Hold timing , Only cell delay, output cell hold requirement and clock uncertainty , Clock skew impact the hold timing

True, clock has no effect on hold time, but I assumed if the setup time is met easier, the tool would have more freedom to insert buffers and fix the hold time violations. Is my assumption wrong?

Yes, Hold fix tool try to fix violation above 0.0. Some of the path may exactly met at slack 0.0 which will not be improved further. If you want to improve the Hold margin, one option is add additional hold margin using set_clock_uncertainty command. set_clock_uncertainty -rise_from [get_clocks {.. }] -rise_to [get_clocks {..}] -hold <Hold margin>

'SYNTH_CLOCK_UNCERTAINITY' was already set to "0.25" (default). Also it seems the OpenLane steps 1-3 issued

set_clock_uncertainty

with the correct value and clock net. Would including it in my sdc make any difference? I also see this in OpenLane configuration README: "SYNTH_CLOCK_UNCERTAINITY: Specifies a value for the clcok uncertainity in the pre-CTS stages" Does this mean it is discarded in the subsequent (post-CTS) stages?

@User if you're sure about timing constraints SYNTH_CLOCK_UNCERTAINITY can be updated as per your need in your config.tcl