evidence · 6.5 KiB

cpu0-identity-trial.md

Committed at docs/evidence/second-core/cpu0-identity-trial.md. This is what grounds the requirements that cite it.

Open raw source

CPU0 SPL identity trial — preparation and reference procedure

This is a bounded diagnostic for the Linux SMP plan, not a second-core application. It reads physical CPU0’s mhartid and misa in the existing SPL handoff before CPU1 is released; afterward stage 1 boots physical CPU1 as usual and CPU0 parks. The bounded trial was performed on 2026-09-27, then the original SPL slots were restored and normal boot verified; see the physical trial evidence. The procedure below records the prepared external-reader route; the actual operator used the running Linux raw slots with an off-board backup and an external reader available as fallback.

Candidate built on 2026-09-26 from pinned U-Boot SDK revision 1104236db4d1e47873bd68924f912747b820228c:

nix build .#stage1-cpu0-identity-probe --no-link --print-out-paths --max-jobs 1 --cores 4
  /nix/store/6y6v6vyg37r04jjj5057j8yfisxayf69-k230-stage1-packaging
candidate fn_u-boot-spl.bin: 223412 bytes
  SHA256 a4c1ae6a038bb3e7326b375f15431c498db5c185dd192cdb4391bd3b265d1cc9
candidate fn_ug_u-boot.bin: 385932 bytes
  SHA256 e0b5787f3e68267f997ca61d45afaf8945e6fe9be775cc6165f981978cf3f1bb
normal fn_ug_u-boot.bin: same SHA256 and `cmp` exit 0
normal .#uboot-k230 derivation: /nix/store/kac0dycgcny0m9yapbhmdydyhps6a75q-uboot-k230_canmv_v3_defconfig-riscv64-unknown-linux-gnu-2022.10.drv
normal .#stage1-packaging derivation: /nix/store/v4dk5d7wssq0rjsvzv6rqcmvsdfrmqpk-k230-stage1-packaging.drv
probe .#stage1-cpu0-identity-probe derivation: /nix/store/16in71n35zplf9rkdcydhjv0686w6pkz-k230-stage1-packaging.drv
wrapper first four bytes: 4b 32 33 30 (ASCII K230)

The SPL wrapper begins with the K230 firmware magic, built by the existing nix/stage1.nix packaging. Its 223412 bytes fit in either 512 KiB raw SPL slot; the underlying SPL binary is 222880 bytes against the 524288-byte CONFIG_SPL_SIZE_LIMIT. The trial package differs from normal stage 1 only in fn_u-boot-spl.bin; do not write fn_ug_u-boot.bin, env, DT, OpenSBI, kernel or rootfs. nix/stage1.nix:176-180 gives the two SPL offsets as 1048576 and 1572864 bytes (1 MiB and 1.5 MiB).

Prepared external-reader procedure

Use an external SD-card reader; U-Boot ums cannot recover an SPL that prevents U-Boot from starting. Record the current card’s full by-id path, sector count, model, and serial before removal, and confirm the same device in the reader. Preserve the current known-good boot state and external power/serial arrangement. Substitute only the confirmed card path and its known sector count below; stop if either identity or size differs, if any partition is mounted, or if the backup/readback hashes differ. The card reader is a recovery route even when no second physical card is available.

set -euo pipefail
card=/dev/disk/by-id/REPLACE_WITH_CONFIRMED_WHOLE_CARD
expected_sectors=REPLACE_WITH_CONFIRMED_SECTOR_COUNT
candidate=/nix/store/6y6v6vyg37r04jjj5057j8yfisxayf69-k230-stage1-packaging/fn_u-boot-spl.bin
test -b "$card"
test "$(lsblk -dnro TYPE "$card")" = disk
test "$(sudo blockdev --getsz "$card")" -eq "$expected_sectors"
test -z "$(lsblk -nrpo MOUNTPOINTS "$card" | tr -d '[:space:]')"
test "$(sha256sum "$candidate" | cut -d' ' -f1)" = a4c1ae6a038bb3e7326b375f15431c498db5c185dd192cdb4391bd3b265d1cc9
lsblk -o NAME,SIZE,MODEL,SERIAL,TRAN,MOUNTPOINTS "$card"
sudo dd if="$card" of=cpu0-trial-original-slot-a.bin bs=512K skip=2 count=1 status=none
sudo dd if="$card" of=cpu0-trial-original-slot-b.bin bs=512K skip=3 count=1 status=none
sha256sum cpu0-trial-original-slot-a.bin cpu0-trial-original-slot-b.bin
sha256sum cpu0-trial-original-slot-a.bin cpu0-trial-original-slot-b.bin > cpu0-trial-original-SHA256SUMS
dd if="$candidate" of=cpu0-trial-slot.bin bs=512K conv=sync status=none
test "$(stat -c %s cpu0-trial-slot.bin)" -eq 524288
sha256sum cpu0-trial-slot.bin

Keep the two backed-up slot files and their recorded hashes outside the card being changed. Confirm that they can be read before writing. Then, with the operator still holding the board session, write only the two SPL slots and read them back before booting:

set -euo pipefail
test -z "$(lsblk -nrpo MOUNTPOINTS "$card" | tr -d '[:space:]')"
sha256sum -c cpu0-trial-original-SHA256SUMS
sudo dd if=cpu0-trial-slot.bin of="$card" bs=512K seek=2 count=1 conv=notrunc,fsync status=progress
sudo dd if=cpu0-trial-slot.bin of="$card" bs=512K seek=3 count=1 conv=notrunc,fsync status=progress
sudo dd if="$card" of=cpu0-trial-readback-a.bin bs=512K skip=2 count=1 status=none
sudo dd if="$card" of=cpu0-trial-readback-b.bin bs=512K skip=3 count=1 status=none
cmp cpu0-trial-slot.bin cpu0-trial-readback-a.bin
cmp cpu0-trial-slot.bin cpu0-trial-readback-b.bin

Only after both comparisons pass, reinsert and boot while the operator captures the entire early serial log. Acceptance for this diagnostic is one CPU0_SPL_IDENTITY mhartid=... misa=... line, then the existing CPU1 OpenSBI banner and normal one-CPU Linux boot. Missing marker, a hang, or another boot difference is a failed trial and triggers restoration; it is not a Linux SMP success. A CPU1 M-mode CSR measurement may still be needed because OpenSBI’s boot-HART ID is the firmware’s logical ID.

Restore the original two 512 KiB slots through the external reader, with the same verified card identity and sector count, after the capture regardless of outcome. Then read back and compare the exact original bytes:

set -euo pipefail
test -b "$card"
test "$(lsblk -dnro TYPE "$card")" = disk
test "$(sudo blockdev --getsz "$card")" -eq "$expected_sectors"
test -z "$(lsblk -nrpo MOUNTPOINTS "$card" | tr -d '[:space:]')"
sha256sum -c cpu0-trial-original-SHA256SUMS
lsblk -o NAME,SIZE,MODEL,SERIAL,TRAN,MOUNTPOINTS "$card"
sudo dd if=cpu0-trial-original-slot-a.bin of="$card" bs=512K seek=2 count=1 conv=notrunc,fsync status=progress
sudo dd if=cpu0-trial-original-slot-b.bin of="$card" bs=512K seek=3 count=1 conv=notrunc,fsync status=progress
sudo dd if="$card" of=cpu0-trial-restored-a.bin bs=512K skip=2 count=1 status=none
sudo dd if="$card" of=cpu0-trial-restored-b.bin bs=512K skip=3 count=1 status=none
cmp cpu0-trial-original-slot-a.bin cpu0-trial-restored-a.bin
cmp cpu0-trial-original-slot-b.bin cpu0-trial-restored-b.bin

Record UTC timestamps, current system/store path, both slot hashes before and after, full serial output with secrets redacted, and the result in a separate physical evidence file. The trial establishes CPU0’s sampled CSR values only. It does not establish a usable unique SBI hart ID, PLIC/ACLINT routing, cross-core cache coherency, or two-CPU Linux scheduling.