ENTROPY_SRC DV document

Goals

  • DV
    • Verify all ENTROPY_SRC 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 ENTROPY_SRC design features, please see the ENTROPY_SRC HWIP technical specification.

Testbench architecture

ENTROPY_SRC 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/entropy_src/dv/tb/tb.sv. It instantiates the ENTROPY_SRC DUT module hw/ip/entropy_src/rtl/entropy_src.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 entropy_src_env_pkg. Some of them in use are:

typedef bit [RNG_BUS_WIDTH-1:0] rng_val_t;

typedef enum { BOOT, STARTUP, CONTINUOUS, HALTED } entropy_phase_e;

// A function to predict the current state (or phase) of the entropy_src DUT
// based on the current setting and number of seeds generated
function automatic entropy_phase_e convert_seed_idx_to_phase(int seed_idx,
                                                             bit fips_enable,
                                                             bit fw_ov_insert);

TL_agent

ENTROPY_SRC 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 ENTROPY_SRC device.

Rng_agent

Entropy_src testbench instantiates this PUSH_pull_agent(/hw/dv/sv/push_pull_agent/doc/) which models the rng source.

Csrng_agent

Entropy_src testbench instantiates this push_PULL_agent(/hw/dv/sv/push_pull_agent/doc/) which models the csrng module.

UVM RAL Model

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

It can be created manually (separately) by running make in the hw/ area.

Stimulus strategy

Test sequences

All test sequences reside in hw/ip/entropy_src/dv/env/seq_lib. The entropy_src_base_vseq virtual sequence is extended from cip_base_vseq and serves as a starting point. All test sequences are extended from entropy_src_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:

  • entropy_src_init: Initialize the ENTROPY_SRC 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 entropy_src_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
  • rng_fifo, csrng_fifo: The rng_fifo provides transaction items from the predictor and the csrng_fifo provide actual post-entropy_src transaction items to compare

Assertions

  • TLUL assertions: The tb/entropy_src_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:

$ cd hw/ip/entropy_src/dv
$ make TEST_NAME=entropy_src_smoke

Testplan

Testpoints

Milestone Name Tests Description
V1 smoke entropy_src_smoke

Enable entropy_src, wait for interrupt, verify entropy.

V1 csr_hw_reset entropy_src_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 entropy_src_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 entropy_src_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 entropy_src_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_reset entropy_src_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.
V1 regwen_csr_and_corresponding_lockable_csrentropy_src_csr_rw
entropy_src_csr_aliasing

Verify regwen CSR and its corresponding lockable CSRs.

  • Randomly access all CSRs
  • Test when regwen CSR is set, its corresponding lockable CSRs become read-only registers

Note:

  • If regwen CSR is HW read-only, this feature can be fully tested by common CSR tests - csr_rw and csr_aliasing.
  • If regwen CSR is HW updated, a separate test should be created to test it.

This is only applicable if the block contains regwen and locakable CSRs.

V2 firmware entropy_src_smoke
entropy_src_fw_ov
entropy_src_rng

Verify ability to access entropy register based on value of efuse input Verify sw_regupd, me_regwen bits enables/disables write access to control registers Verify control registers are read-only while DUT is enabled Verify registers at End-Of-Test

V2 firmware_mode entropy_src_fw_ov

Verify health_checks aren't active Verify bypass active Verify read FIFO

  • Random FIFO depths
V2 rng_mode entropy_src_rng

Verify rng entropy

  • Random FIFO depths
  • Random rates
  • Verify single_bit_mode for all bit_selector values Verify FIPS bits match predicted
V2 health_checks entropy_src_rng

Verify AdaptProp, RepCnt, RepCntSym, Bucket, Markov health check results match predicted.

  • Generate passing and failing raw entropy streams
  • Random window sizes
  • Default and random hi/lo bypass/fips thresholds
  • Enables/fail counts/clears
  • Verify hi/lo bypass/fips watermarks
  • Verify External health check behaves as predicted
  • Verify outputs match internal reg values/entropy bus
  • Pulse inputs and verify captured
  • Verify health testing stops when no demand for entropy
V2 conditioning entropy_src_rng

Verify genbits seeds in bypass mode as predicted. Verify genbits seeds after shah3 conditioning as predicted.

V2 interrupts entropy_src_intr

Verify es_entropy_valid interrupt asserts as predicted. Verify es_health_test_failed interrupt asserts as predicted. Verify es_fifo_err interrupt asserts as predicted.

V2 alerts entropy_src_alert

Verify es_alert_count_met asserts as expected.

V2 stress_all entropy_src_stress_all

Combine the individual test points while injecting TL errors and running CSR tests in parallel.

V2 fifo_errs entropy_src_err

Verify they never occur with asserts

V2 intr_test entropy_src_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 entropy_src_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 entropy_src_tl_errors

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

V2 tl_d_illegal_access entropy_src_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
    • write with instr_type = True
V2 tl_d_outstanding_access entropy_src_csr_hw_reset
entropy_src_csr_rw
entropy_src_csr_aliasing
entropy_src_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 entropy_src_csr_hw_reset
entropy_src_csr_rw
entropy_src_csr_aliasing
entropy_src_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.

V2S tl_intg_err entropy_src_tl_intg_err
entropy_src_sec_cm

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.
  • Inject a fault at the onehot check in u_reg.u_prim_reg_we_check and verify the corresponding fatal alert occurs
V2S sec_cm_config_regwen

Verify the countermeasure(s) CONFIG.REGWEN.

V2S sec_cm_config_mubi

Verify the countermeasure(s) CONFIG.MUBI.

V2S sec_cm_config_redun

Verify the countermeasure(s) CONFIG.REDUN.

V2S sec_cm_intersig_mubi

Verify the countermeasure(s) INTERSIG.MUBI.

V2S sec_cm_main_sm_fsm_sparse

Verify the countermeasure(s) MAIN_SM.FSM.SPARSE.

V2S sec_cm_ack_sm_fsm_sparse

Verify the countermeasure(s) ACK_SM.FSM.SPARSE.

V2S sec_cm_rng_bkgn_chk

Verify the countermeasure(s) RNG.BKGN_CHK.

V2S sec_cm_ctr_redun

Verify the countermeasure(s) CTR.REDUN.

V2S sec_cm_ctr_local_esc

Verify the countermeasure(s) CTR.LOCAL_ESC.

V2S sec_cm_esfinal_rdata_bus_consistency

Verify the countermeasure(s) ESFINAL_RDATA.BUS.CONSISTENCY.

V2S sec_cm_tile_link_bus_integrity

Verify the countermeasure(s) TILE_LINK.BUS.INTEGRITY.

V3 stress_all_with_rand_reset entropy_src_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.

Covergroups

Name Description
alert_cnt_cg

Covers a range of values (1 to 16, plus > 16) for ALERT_THRESHOLD

err_test_cg

Covers that all health test failures, fatal errors, all counter errors and all error codes of entropy_src have been tested. Individual config settings that will be covered include:

  • which_ht_fail (0 to 3), 4 possible health test fails, repcnt, adaptp, bucket and markov test fails
  • which_ht (0 to 1), adaptp and markov health tests have low and high tests
  • which_fatal_err (0 to 5), 6 fatal errors, esrng, observe, esfinal fifo errors, main state machine, ack state machine errors, and counter error
  • which_fifo_err (0 to 2), fifo has write, read and state errors
  • which_fifo (0 to 2), esrng, observe and esfinal fifos
  • which_cntr (0 to 5), 6 possible counter errors, window counter, repcnt ht counter, repcnts ht counter, adaptive proportion ht counter, bucket ht counter and markov ht counter
  • which_err_code (0 to 17), ERR_CODE has 9 fields, plus 9 ERR_CODE_TEST bits test
  • which_invalid_mubi (0 to 8), 9 possible invalid mubi value fields
  • which_cntr_replicate (0 to RNG_BUS_WIDTH-1), reptcnt, adaptp, markov health tests have RNG_BUS_WIDTH copies of counters
  • which_bin (0 to 2RNG_BUS_WIDTH-1), bucket health test has 2RNG_BUS_WIDTH copies of counters
fw_ov_output_cg

Covers that data output is observed at the fw_ov_rd_data CSE interface for all possible modes of operation, including:

  • CONF.FIPS_ENABLE (True and False)
  • CONF.ENTROPY_DATA_REG_ENABLE (True and False)
  • CONF.THRESHOLD_SCOPE (True and False)
  • CONF.RNG_BIT_ENABLE (True and False)
  • CONF.RNG_BIT_SEL (0 to 3)
  • ENTROPY_CONTROL.ES_ROUTE (True and False), If True, data must be observed at the ENTROPY_DATA CSR. If False, data must be observed at the CSRNG port.
  • ENTROPY_CONTROL.ES_TYPE (True and False)
  • FW_OV_MODE (True or False)
  • FW_OV_ENTROPY_INSERT (True or False) Since the scoreboard permits data to be dropped by the entropy source we must explicitly confirm that the data is observed at this output for all possible configurations.
ht_threshold_cg

Covers a range of values for all health test thresholds. For each test we need:

  • Test: ADAPTB, BUCKET, MARKOV, REPCNT, and REPTCNTS. No cross between tests. EXT HT not used or covered for ES
  • Mode: Bypass or FIPS
  • window_size: {384, 512, 1024, 2048, 4096, plus other non-powers of two}
  • Range buckets:
    • For the ADAPTP, BUCKET and MARKOV test, the configuration sets the thresholds in units the likelihood of HT failure (i.e., sigma values): covers minimum, maximum, 0-2 sigma, 1-2 sigma, 2-4 sigma, 4-6 sigma, > 6 sigma
    • For REPCNT and REPCNTS tests covers buckets of 0-32, 32-64, 64-96, > 96
  • Result: HT Pass and Failure
observe_fifo_thresh_cg

Covers a range of values (1-64) for OBSERVE_FIFO_THRESH. Interrupts and data must be observed for all values.

regwen_val_when_new_value_written_cg

Cover each lockable reg field with these 2 cases:

  • When regwen = 1, a different value is written to the lockable CSR field, and a read occurs after that.
  • When regwen = 0, a different value is written to the lockable CSR field, and a read occurs after that.

This is only applicable if the block contains regwen and locakable CSRs.

seed_output_entropy_data_cg

Covers that data output is observed at the entropy_data CSR interfaces for all possible modes of operation, including:

  • CONF.FIPS_ENABLE (True and False)
  • CONF.THRESHOLD_SCOPE (True and False)
  • CONF.RNG_BIT_ENABLE (True and False)
  • CONF.RNG_BIT_SEL (0 to 3)
  • ENTROPY_CONTROL.ES_TYPE (True and False)
  • FW_OV_MODE (True or False)
  • FW_OV_ENTROPY_INSERT (True or False) In addition to the above, the following settings are illegal when sampling on the this covergroup, and merit the creation of illegal_bins
  • ENTROPY_CONTROL.ES_ROUTE = False
  • CONF.ENTROPY_DATA_REG_ENABLE = False

Since the scoreboard permits data to be dropped or rejected by the entropy source we must explicitly confirm that the data is observed at the outputs for all possible configurations.

seed_output_hw_cg

Covers that data output is observed at the CSRNG HW interface for all possible modes of operation, including:

  • CONF.FIPS_ENABLE (True and False)
  • CONF.ENTROPY_DATA_REG_ENABLE (True and False)
  • CONF.THRESHOLD_SCOPE (True and False)
  • CONF.RNG_BIT_ENABLE (True and False)
  • CONF.RNG_BIT_SEL (0 to 3)
  • ENTROPY_CONTROL.ES_TYPE (True and False)
  • FW_OV_MODE (True or False)
  • FW_OV_ENTROPY_INSERT (True or False) In addition to the above, the following settings are illegal when sampling on this covergroup, and merit the creation of illegal_bins
  • ENTROPY_CONTROL.ES_ROUTE = True

Since the scoreboard permits data to be dropped or rejected by the entropy source we must explicitly confirm that the data is observed at the outputs for all possible configurations.

sw_update_cg

Covers that the TB has attempted to update DUT configurations while the module is enabled, to ensure that the sw_regupd CSR is working

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.

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