CSRNG DV document

Goals

  • DV
    • Verify all CSRNG IP features by running dynamic simulations with a SV/UVM based testbench
    • Develop and run all tests based on the testplan below towards closing code and functional coverage on the IP and all of its sub-modules
  • FPV
    • Verify TileLink device protocol compliance with an SVA based testbench

Current status

Design features

For detailed information on CSRNG design features, please see the CSRNG HWIP technical specification.

Testbench architecture

CSRNG testbench has been constructed based on the CIP testbench architecture.

Block diagram

Block diagram

Top level testbench

Top level testbench is located at hw/ip/csrng/dv/tb/tb.sv. It instantiates the CSRNG DUT module hw/ip/csrng/rtl/csrng.sv. In addition, it instantiates the following interfaces, connects them to the DUT and sets their handle into uvm_config_db:

Common DV utility components

The following utilities provide generic helper tasks and functions to perform activities that are common across the project:

Global types & methods

All common types and methods defined at the package level can be found in csrng_env_pkg. Some of them in use are:

TL_agent

CSRNG testbench instantiates (already handled in CIP base env) tl_agent which provides the ability to drive and independently monitor random traffic via TL host interface into CSRNG device.

Entropy_src_agent

CSRNG testbench instantiates this push_pull_agent(/hw/dv/sv/push_pull_agent/README/) which models the ENTROPY_SRC module.

Csrng_agent

UVM RAL Model

The CSRNG RAL model is created with the ralgen FuseSoC generator script automatically when the simulation is at the build stage.

It can be created manually by invoking regtool:

Stimulus strategy

Test sequences

All test sequences reside in hw/ip/csrng/dv/env/seq_lib. The csrng_base_vseq virtual sequence is extended from cip_base_vseq and serves as a starting point. All test sequences are extended from csrng_base_vseq. It provides commonly used handles, variables, functions and tasks that the test sequences can simple use / call. Some of the most commonly used tasks / functions are as follows:

  • csrng_init: Initialize the CSRNG module from the randomized environment variables in the config.

Functional coverage

To ensure high quality constrained random stimulus, it is necessary to develop a functional coverage model. The following covergroups have been developed to prove that the test intent has been adequately met:

  • common covergroup for interrupts hw/dv/sv/cip_lib/cip_base_env_cov.sv: Cover interrupt value, interrupt enable, intr_test, interrupt pin

Self-checking strategy

Scoreboard

The csrng_scoreboard is primarily used for end to end checking. It creates the following analysis ports to retrieve the data monitored by corresponding interface agents:

  • tl_a_chan_fifo, tl_d_chan_fifo: These 2 fifos provide transaction items at the end of Tilelink address channel and data channel respectively
  • entropy_src_fifo, genbits_fifo: The entropy_src_fifo provides transaction items from the predictor and the genbits_fifo provide actual post-entropy_src transaction items to compare

Assertions

  • TLUL assertions: The tb/csrng_bind.sv binds the tlul_assert assertions to the IP to ensure TileLink interface protocol compliance.
  • Unknown checks on DUT outputs: The RTL has assertions to ensure all outputs are initialized to known values after coming out of reset.

Building and running tests

We are using our in-house developed regression tool for building and running our tests and regressions. Please take a look at the link for detailed information on the usage, capabilities, features and known issues. Here’s how to run a smoke test:

$ $REPO_TOP/util/dvsim/dvsim.py $REPO_TOP/hw/ip/csrng/dv/csrng_sim_cfg.hjson -i csrng_smoke

Testplan

Testpoints

Milestone Name Tests Description
V1 smoke csrng_smoke

Enable csrng, send instantiate/generate cmds, verify 0-seed genbits/interrupt.

V1 csr_hw_reset csrng_csr_hw_reset

Verify the reset values as indicated in the RAL specification.

  • Write all CSRs with a random value.
  • Apply reset to the DUT as well as the RAL model.
  • Read each CSR and compare it against the reset value. it is mandatory to replicate this test for each reset that affects all or a subset of the CSRs.
  • It is mandatory to run this test for all available interfaces the CSRs are accessible from.
  • Shuffle the list of CSRs first to remove the effect of ordering.
V1 csr_rw csrng_csr_rw

Verify accessibility of CSRs as indicated in the RAL specification.

  • Loop through each CSR to write it with a random value.
  • Read the CSR back and check for correctness while adhering to its access policies.
  • It is mandatory to run this test for all available interfaces the CSRs are accessible from.
  • Shuffle the list of CSRs first to remove the effect of ordering.
V1 csr_bit_bash csrng_csr_bit_bash

Verify no aliasing within individual bits of a CSR.

  • Walk a 1 through each CSR by flipping 1 bit at a time.
  • Read the CSR back and check for correctness while adhering to its access policies.
  • This verify that writing a specific bit within the CSR did not affect any of the other bits.
  • It is mandatory to run this test for all available interfaces the CSRs are accessible from.
  • Shuffle the list of CSRs first to remove the effect of ordering.
V1 csr_aliasing csrng_csr_aliasing

Verify no aliasing within the CSR address space.

  • Loop through each CSR to write it with a random value
  • Shuffle and read ALL CSRs back.
  • All CSRs except for the one that was written in this iteration should read back the previous value.
  • The CSR that was written in this iteration is checked for correctness while adhering to its access policies.
  • It is mandatory to run this test for all available interfaces the CSRs are accessible from.
  • Shuffle the list of CSRs first to remove the effect of ordering.
V1 csr_mem_rw_with_rand_resetcsrng_csr_mem_rw_with_rand_reset

Verify random reset during CSR/memory access.

  • Run csr_rw sequence to randomly access CSRs
  • If memory exists, run mem_partial_access in parallel with csr_rw
  • Randomly issue reset and then use hw_reset sequence to check all CSRs are reset to default value
  • It is mandatory to run this test for all available interfaces the CSRs are accessible from.
V2 firmware

Verify ability to access genbits register based on value of efuse input. Verify regen bit disables write access of control registers. Verify registers at End-Of-Test.

V2 interrupts

Verify cs_cmd_req_done interrupt asserts/clears as predicted. Verify cs_entropy_req interrupt asserts/clears as predicted. Verify cs_hw_inst_exc interrupt asserts/clears as predicted. Verify cs_fifo_err interrupt asserts/clears as predicted. Verify fifo error status bits are set as predicted.

V2 cmds

Verify all SW app csrng commands req/status behave as predicted. Verify all HW app csrng commands req/status behave as predicted. Verify above for all valid values of acmd, clen, flags, glen. Verify for multiple hw app interfaces running in parallel. Verify sw/hw app interfaces running in parallel.

V2 life cycle

Verify lifecycle hardware debug mode enables AES bypass, reading CSRNG internal state. Verify CSRNG internal state for all csrng/genbits operations.

V2 genbits

Verify genbits generated as predicted. Verify fips bits is passed through properly. Verify for multiple hw app interfaces running in parallel.

V2 stress_all

Combine the other individual testpoints while injecting TL errors and running CSR tests in parallel.

V2 intr_test csrng_intr_test

Verify common intr_test CSRs that allows SW to mock-inject interrupts.

  • Enable a random set of interrupts by writing random value(s) to intr_enable CSR(s).
  • Randomly "turn on" interrupts by writing random value(s) to intr_test CSR(s).
  • Read all intr_state CSR(s) back to verify that it reflects the same value as what was written to the corresponding intr_test CSR.
  • Check the cfg.intr_vif pins to verify that only the interrupts that were enabled and turned on are set.
  • Clear a random set of interrupts by writing a randomly value to intr_state CSR(s).
  • Repeat the above steps a bunch of times.
V2 alert_test csrng_alert_test

Verify common alert_test CSR that allows SW to mock-inject alert requests.

  • Enable a random set of alert requests by writing random value to alert_test CSR.
  • Check each alert_tx.alert_p pin to verify that only the requested alerts are triggered.
  • During alert_handshakes, write alert_test CSR again to verify that: If alert_test writes to current ongoing alert handshake, the alert_test request will be ignored. If alert_test writes to current idle alert handshake, a new alert_handshake should be triggered.
  • Wait for the alert handshakes to finish and verify alert_tx.alert_p pins all sets back to 0.
  • Repeat the above steps a bunch of times.
V2 tl_d_oob_addr_access csrng_tl_errors

Access out of bounds address and verify correctness of response / behavior

V2 tl_d_illegal_access csrng_tl_errors

Drive unsupported requests via TL interface and verify correctness of response / behavior. Below error cases are tested bases on the [TLUL spec]({{< relref "hw/ip/tlul/doc/_index.md#explicit-error-cases" >}})

  • TL-UL protocol error cases
    • invalid opcode
    • some mask bits not set when opcode is PutFullData
    • mask does not match the transfer size, e.g. a_address = 0x00, a_size = 0, a_mask = 'b0010
    • mask and address misaligned, e.g. a_address = 0x01, a_mask = 'b0001
    • address and size aren't aligned, e.g. a_address = 0x01, a_size != 0
    • size is greater than 2
  • OpenTitan defined error cases
    • access unmapped address, expect d_error = 1 when devmode_i == 1
    • write a CSR with unaligned address, e.g. a_address[1:0] != 0
    • write a CSR less than its width, e.g. when CSR is 2 bytes wide, only write 1 byte
    • write a memory with a_mask != '1 when it doesn't support partial accesses
    • read a WO (write-only) memory
    • write a RO (read-only) memory
V2 tl_d_outstanding_access csrng_csr_hw_reset
csrng_csr_rw
csrng_csr_aliasing
csrng_same_csr_outstanding

Drive back-to-back requests without waiting for response to ensure there is one transaction outstanding within the TL device. Also, verify one outstanding when back- to-back accesses are made to the same address.

V2 tl_d_partial_access csrng_csr_hw_reset
csrng_csr_rw
csrng_csr_aliasing
csrng_same_csr_outstanding

Access CSR with one or more bytes of data. For read, expect to return all word value of the CSR. For write, enabling bytes should cover all CSR valid fields.

V2 stress_all_with_rand_resetcsrng_stress_all_with_rand_reset

This test runs 3 parallel threads - stress_all, tl_errors and random reset. After reset is asserted, the test will read and check all valid CSR registers.

V3 tl_intg_err csrng_tl_intg_err

Verify that the data integrity check violation generates an alert.

Randomly inject errors on the control, data, or the ECC bits during CSR accesses. Verify that triggers the correct fatal alert.

Covergroups

Name Description
tl_errors_cg

Cover the following error cases on TL-UL bus:

  • TL-UL protocol error cases.
  • OpenTitan defined error cases, refer to testpoint tl_d_illegal_access.
tl_intg_err_cg

Cover all kinds of integrity errors (command, data or both) and cover number of error bits on each integrity check.

tl_intg_err_mem_subword_cg

Cover the kinds of integrity errors with byte enabled write on memory.

Some memories store the integrity values. When there is a subword write, design re-calculate the integrity with full word data and update integrity in the memory. This coverage ensures that memory byte write has been issued and the related design logic has been verfied.