Someone name fmarch asked on https://news.ycombinator.com/item?id=10831601 about verification against the ISA model.
It possibly can be done via a torture tester apparently, https://github.com/ucb-bar/riscv-torture , but taking a quick look I don't think it handles loops, interrupts, floating point instructions etc.
Doesn't seem nearly as thorough as what I've read in ASIC papers on verification. They did (co-simulation?), equivalence, gate-level testing, all kinds of stuff. Plus, you did it for a living so I take your word there. I do hope they have some other stuff somewhere if they're doing tapeouts at 28nm. Hard to imagine unless they just really trust the synthesis and formal verification tools.
The flow is here:
Are those tools and techniques good enough to get first pass if the Chisel output was good enough to start with? Would it work in normal cases until it hits corner cases or has physical failures?
The torture test page said it still needed support for floating point instructions. That kinda says, they did no torture tests of floating point instructions. I wouldn't be happy with that. Same goes for loops. Etc.
You have to think about physical failures as well: the paper mentions various RAMs in the 45 nm processor. You should have BIST for those and Design For Test module/s. Otherwise you have no way to test for defects.