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lowRISC / opentitan

OpenTitan: Open source silicon root of trust
https://www.opentitan.org
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[entropy_src/dv] Find out what happened to the group coverage #20565

Closed h-filali closed 11 months ago

h-filali commented 11 months ago

Description

Between the 16. and the 19. of October something caused the entropy source coverage to plummet to around 30 to 40%. To verify this I ran the following command with a 0.1 reseed-multiplier: ./util/dvsim/dvsim.py hw/ip/entropy_src/dv/entropy_src_sim_cfg.hjson -i all -t vcs --reseed-multiplier 0.1 --cov image

We need to find out what happened and fix this issue.

h-filali commented 11 months ago

image These are the results after a full regression using xcelium with the following command: ./util/dvsim/dvsim.py hw/ip/entropy_src/dv/entropy_src_sim_cfg.hjson -i all -t xcelium --cov Having a closer look at the coverage groups looks like this: image

h-filali commented 11 months ago

Status update: I was able to narrow the issue down to PR #20036 There now seems to be an issue with the random variables of the cfg class. The rand variables don't seem to be random at all. They all seem to have the same value, no matter what seed is used. This likely also affects other IP blocks. In the dashboard the group coverage goes down for several blocks between the nightly regression before and after the above PR.

h-filali commented 11 months ago

Status update #2: The PR mentioned in the previous comment changes seed lengths (e.g. for the simulation and OTP) from 32 to 256 bits and systemverilog can only deal with 32 bit seeds. For this reason xrun just resorts to using a default seed, which means that we weren't really doing directed random tests in the past 1-2 months. This also explains why our coverage went down.

Furthermore, since the seed that is generated for systemverilog is not only used for seeding the systemverilog random variables, we can't just change this seed to 32 bits. A potential solution is to take the lowest 32 bits of the 256 bit seed and use it for the command that is issued when the test is being deployed. If I understand correctly, this happens in Deploy.py. There is a dict called mandatory_cmd_attrs where the data for the deploy command comes from. I'll continue working on this on monday.