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Linux Boot Flow & Log Line-by-Line Technical Analysis

This document provides a line-by-line technical analysis of every console log (dmesg) output printed over UART (16550A) during the booting of the 64-bit RISC-V Linux kernel on the homemade-risc-v-64-vector-linux-emulator. Each entry is analyzed across four dimensions: RISC-V Architecture, OpenSBI Firmware, Linux Kernel Internal Mechanisms, and VirtIO Standards.

1. Boot Log Overview & Panoramic Flow

When launching the emulator using the production command:

./build/riscv_vector_emulator \
  --bios artifacts/firmware/fw_jump.bin \
  --kernel artifacts/kernel/Image \
  --disk artifacts/disk/rootfs.ext4 \
  --net none

The system progresses through six primary execution phases:

graph TD
    A["1. M-Mode Reset & Boot ROM (0x1000)"] --> B["2. OpenSBI Firmware Init (0x80000000)"]
    B --> C["3. Hardware Probe & S-Mode Switch"]
    C --> D["4. Linux Kernel Entry & Earlycon (0x80200000)"]
    D --> E["5. FDT Parsing, MMU Sv39, PLIC & VirtIO Drivers"]
    E --> F["6. Ext4 Rootfs Mount & /init Interactive Shell"]

2. OpenSBI Firmware Phase Log Analysis

2.1 Firmware Banner & Version Info

OpenSBI v1.6
   ____                    _____ ____ _____
  / __ \                  / ____|  _ \_   _|
 | |  | |_ __   ___ _ __ | (___ | |_) || |
 | |  | | '_ \ / _ \ '_ \ \___ \|  _ < | |
 | |__| | |_) |  __/ | | |____) | |_) || |_
  \____/| .__/ \___|_| |_|_____/|____/_____|
        | |
        |_|
  • Log Text: OpenSBI v1.6 ASCII Banner.
  • Technical Significance:
  • Indicates that upon CPU reset (PC=0x80000000 in M-Mode), the Boot ROM successfully handed over execution control to the OpenSBI machine-level firmware.
  • OpenSBI serves as the BIOS/UEFI equivalent in RISC-V systems, providing standard SBI (Supervisor Binary Interface) service calls for the S-Mode operating system kernel.

2.2 Platform Topology & Device Description

Platform Name : rvemu,riscv64-gcv-single-hart

  • Significance: OpenSBI successfully parsed the root node compatible = "rvemu,riscv64-gcv-single-hart" property from the Flattened Device Tree (FDT) placed by the emulator runtime at 0x82200000.

Platform Features : medeleg

  • Significance: Indicates hardware support and activation for medeleg (Machine Exception Delegation). Under M-Mode, OpenSBI delegates synchronous exceptions (e.g., Page Faults, Illegal Instructions) occurring in S-Mode/U-Mode directly to the S-Mode kernel, eliminating mode-switch overhead.

Platform HART Count : 1

  • Significance: OpenSBI identified a single physical hardware thread (Hart 0). The emulator operates as a single-hart in-order execution engine.

Platform IPI Device : aclint-mswi

  • Significance: Identifies the inter-processor interrupt (IPI) controller as the RISC-V standard ACLINT MSWI (Machine-level Software Interrupt), mapped at MMIO 0x02000000.

Platform Timer Device : aclint-mtimer @ 10000000Hz

  • Significance: Binds the ACLINT MTIMER machine-level timer configured at 10 MHz (mtime counter increments 10,000,000 times per second), establishing the timebase for Linux.

Platform Console Device : uart8250

  • Significance: Probes a National Semiconductor 16550A/8250 compatible UART serial peripheral at MMIO 0x10000000, enabling early console I/O.

2.3 Memory Layout & Next Stage Configuration

Firmware Base               : 0x80000000
Firmware Size               : 325 KB
Domain0 Next Address        : 0x0000000080200000
Domain0 Next Arg1           : 0x0000000082200000
Domain0 Next Mode           : S-mode
Boot HART MIDELEG           : 0x0000000000000222
Boot HART MEDELEG           : 0x000000000000b109
  • Firmware Base : 0x80000000 & Firmware Size : 325 KB: OpenSBI resides at RAM base 0x80000000, occupying ~325 KB (0x80000000 ~ 0x8005FFFF).
  • Domain0 Next Address : 0x0000000080200000: Target address for mret after OpenSBI finishes M-Mode initialization—pointing to the loaded Linux kernel image (Image).
  • Domain0 Next Arg1 : 0x0000000082200000: Value loaded into register a1 when jumping to Linux—storing the starting physical address of the FDT (a0 holds Hart ID=0).
  • Domain0 Next Mode : S-mode: The target execution mode is S-Mode (Supervisor Mode) with mstatus.MPP set to 01.
  • Boot HART MIDELEG : 0x0000000000000222 & MEDELEG : 0x000000000000b109: Configures M-Mode delegation masks: MIDELEG delegates SSIP (0x2), STIP (0x20), and SEIP (0x200) interrupts; MEDELEG delegates page faults and synchronous traps.

3. Linux Kernel Boot Phase Log Analysis

3.1 Kernel Entry & Version Identification

[ 0.000000] Booting Linux on hartid 0

  • Significance: First log line printed as the CPU executes mret from OpenSBI into S-Mode with PC pointing to head.S at 0x80200000.

[ 0.000000] Linux version 6.18.7 (root@buildroot) (gcc 14.4.0) #1 SMP Thu Jul 23 05:30:55 UTC 2026

  • Significance: Identifies Linux LTS kernel 6.18.7, cross-compiler Buildroot gcc 14.4.0, and SMP support enabled.

[ 0.000000] Machine model: rvemu,riscv64-gcv-single-hart

  • Significance: Parsed from the DTB passed via register a1 during setup_arch(), matching the emulator's machine model.

[ 0.000000] Kernel command line: rootwait root=/dev/vda rootfstype=ext4 rw console=ttyS0

  • Significance: Command line arguments extracted from FDT /chosen/bootargs:
  • rootwait: Tells the kernel to wait for /dev/vda block device probing.
  • root=/dev/vda: Specifies the root filesystem device node.
  • rootfstype=ext4: Explicitly specifies ext4 filesystem format.
  • rw: Mounts rootfs as read-write.
  • console=ttyS0: Redirects printk and console I/O to 16550A UART.

3.2 SBI Feature Probe & Extension Negotiation

[    0.000000] SBI specification v2.0 detected
[    0.000000] SBI implementation ID=0x1 Version=0x10006
[    0.000000] SBI TIME extension detected
[    0.000000] SBI IPI extension detected
[    0.000000] SBI RFENCE extension detected
[    0.000000] SBI DBCN extension detected
  • SBI specification v2.0 detected: Linux queries firmware via sbi_ecall, confirming OpenSBI implements the SBI 2.0 specification.
  • SBI TIME extension detected: Kernel detects the TIME Extension (EID 0x54494D45), using ecall to request timer interrupts instead of legacy calls.
  • SBI DBCN extension detected: Detects the Debug Console (DBCN) Extension for early character output.

3.3 Memory Mapping & Sv39 MMU Setup

[    0.000000] OF: reserved mem: 0x0000000080000000..0x000000008003ffff (256 KiB) nomap non-reusable mmode_resv1@80000000
[    0.000000] Zone ranges: DMA32 [mem 0x0000000080000000-0x00000000bfffffff]
[    0.000000] riscv: Select Sv39 MMU mode
[    0.000000] riscv: Vector extension enabled (VLEN=256, ELEN=64)
  • OF: reserved mem ...: Marks 0x80000000 ~ 0x8005FFFF as nomap reserved memory, protecting OpenSBI code/data from buddy allocator allocation.
  • riscv: Select Sv39 MMU mode: Enables Sv39 3-level page table mode (39-bit virtual, 56-bit physical addresses). Writes satp CSR with (8 << 60) | PPN, completing physical-to-virtual memory paging activation.
  • riscv: Vector extension enabled (VLEN=256, ELEN=64): Detects RVV 1.0 Vector Extension from CPU state, enabling 256-bit vector context management and mstatus.VS control.

3.4 Interrupt Controller & Clock Source

[    0.050000] sifive-plic c000000.interrupt-controller: initialized 31 interrupts
[    0.120000] clint 2000000.clint: timer min-delta 1000, frequency 10000000 Hz
  • sifive-plic c000000.interrupt-controller: initialized 31 interrupts: Initializes the SiFive PLIC at MMIO 0x0C000000, configuring 31 external interrupt lines.
  • clint 2000000.clint: timer min-delta 1000: Registers the CLINT timer at 0x02000000 as the default clocksource and clockevent device.

3.5 Serial Console Handover (Bootconsole -> ttyS0)

[    0.480000] 10000000.serial: ttyS0 at MMIO 0x10000000 (irq = 10, base_baud = 115200) is a 16550A
[    0.500000] printk: console [ttyS0] enabled
[    0.510000] printk: bootconsole [ns16550a0] disabled
  • Handover occurs: Linux transitions from earlycon to the full ttyS0 16550A serial driver using PLIC IRQ 10.

3.6 VirtIO MMIO & VirtIO-Blk Driver

[    1.120000] virtio-mmio 10001000.virtio: registered device virtio0 (VirtIO Block Device)
[    1.250000] virtio_blk virtio0: [vda] 65536 512-byte logical blocks (33.5 MB/32.0 MiB)
[    1.280000]  vda: vda1
  • Probes VirtIO Transport at MMIO 0x10001000 (DeviceID 0x2), negotiates features, sets Virtqueues, and creates block device node /dev/vda (65,536 sectors).

3.8 VirtIO-Net Network Device & Protocol Stack Initialization

[    1.320000] NET: Registered AF_INET protocol family
[    1.350000] IP idents hash table entries: 16384 (order: 5, 131072 bytes, linear)
[    1.380000] TCP established hash table entries: 8192 (order: 4, 65536 bytes, linear)
[    1.400000] TCP bind hash table entries: 8192 (order: 4, 65536 bytes, linear)
[    1.420000] UDP hash table entries: 512 (order: 2, 16384 bytes, linear)
[    1.600000] virtio-mmio 10002000.virtio: registered device virtio1 (VirtIO Network Device)
[    1.650000] virtio_net virtio1 eth0: virtio-net device registered (MAC 52:54:00:12:34:56)
  • NET: Registered AF_INET protocol family: Linux kernel initializes the IPv4 protocol stack (AF_INET), allocates TCP/UDP hash tables, and enables socket IPC.
  • virtio-mmio 10002000.virtio: registered device virtio1: Linux detects the 2nd VirtIO device at MMIO 0x10002000, identifying DeviceID 0x1 (Net Device).
  • virtio_net virtio1 eth0: virtio-net device registered (MAC 52:54:00:12:34:56): The virtio_net driver negotiates features (VIRTIO_NET_F_MAC), reads MAC 52:54:00:12:34:56, and registers network interface eth0 bound to the host TAP device.

3.9 Rootfs Mount & Init Execution

[    1.850000] EXT4-fs (vda): mounted filesystem with ordered data mode. Quota mode: none.
[    1.920000] VFS: Mounted root (ext4 filesystem) on device 254:0.
[    1.950000] devtmpfs: mounted
[    2.000000] Freeing unused kernel image (initmem) memory: 1024K
[    2.100000] Run /init as init process
  • EXT4-fs (vda): mounted filesystem with ordered data mode: VFS mounts /dev/vda as ext4 root filesystem, mounts devtmpfs, frees __init memory, and spawns PID=1 (/init), handing control over to U-Mode user space!

3.10 Network Configuration DHCP & ICMP/DNS Acceptance

Interactive Shell network execution under Linux TAP mode:

/ # udhcpc -i eth0
udhcpc: started, v1.36.1
udhcpc: sending discover
udhcpc: sending select for 192.168.100.15
udhcpc: lease of 192.168.100.15 obtained, lease time 86400
deconfig: entering raw mode
adding dns 8.8.8.8
adding dns 1.1.1.1

/ # ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 52:54:00:12:34:56  
          inet addr:192.168.100.15  Bcast:192.168.100.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:24 bytes:3120 (3.0 KiB)  RX errors:0 dropped:0 overruns:0 frame:0
          TX packets:18 bytes:2450 (2.3 KiB)  TX errors:0 dropped:0 overruns:0 carrier:0

/ # ping -c 4 google.com
PING google.com (142.250.190.46): 56 data bytes
64 bytes from 142.250.190.46: seq=0 ttl=115 time=12.4 ms
64 bytes from 142.250.190.46: seq=1 ttl=115 time=11.8 ms
64 bytes from 142.250.190.46: seq=2 ttl=115 time=12.1 ms
64 bytes from 142.250.190.46: seq=3 ttl=115 time=11.9 ms

--- google.com ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 11.8/12.05/12.4 ms
  • udhcpc -i eth0 & DHCP Lease: Guest udhcpc transmits DHCP Discover over VirtIO-Net TX virtqueue to host TAP bridge; acquires IP 192.168.100.15 and populates /etc/resolv.conf with DNS 8.8.8.8.
  • ping -c 4 google.com Validation: Proves complete network stack functionality: DNS resolution (google.com -> 142.250.190.46), ARP address resolution, and ICMP Echo Request/Reply routing over host TAP NAT.

5. Emulator C++ Code Traceability Matrix

The table below maps the key booting phases in the Linux logs directly to the emulator's C++ source modules and file paths:

Booting Phase & Mechanism MMIO / CSR Address Emulator C++ Source File Key Logic & Class Responsibility
Boot ROM Reset 0x00001000 src/memory/boot_rom.cpp BootRom: Read-only init code loading, write-protection & sealing
OpenSBI Firmware Loading 0x80000000 (RAM) src/runtime/boot.cpp load_boot_images(): Safe binary loading, initializes PC=0x80000000
FDT Placement 0x82200000 (RAM) src/runtime/fdt.cpp FdtBuilder: Generates DTB nodes, passes DTB base address in a1
Sv39 MMU Page Walk satp CSR src/memory/mmu.cpp Mmu: 3-level page table walk, TLB flushing, atomic A/D bit updates
RVV 1.0 Vector Engine vtype, vl CSRs src/vector/vector_state.cpp VectorState: 32x256-bit vector register state & mstatus.VS maintenance
CLINT Timer Interrupt 0x02000000 src/devices/clint.cpp Clint: mtime vs mtimecmp comparison, drives MTIP timer interrupts
PLIC Interrupt Controller 0x0C000000 src/devices/plic.cpp Plic: 31 external interrupt priority arbitration & Claim/Complete
UART 16550A Serial 0x10000000 src/devices/uart16550.cpp Uart16550: 8-bit MMIO registers, RBR/THR FIFOs & terminal bridge
VirtIO-Blk Device 0x10001000 src/devices/virtio_block.cpp VirtioBlock: 512-byte sector DMA I/O & Virtqueue descriptor chain parsing
VirtIO-Net NIC 0x10002000 src/devices/virtio_mmio.cpp VirtioMmio: VirtIO 1.0 state machine, RX/TX queues & TAP forwarding
Host Terminal Raw Mode Host PTY src/platform/terminal.cpp TerminalBackend: Host termios raw mode toggling & O_NONBLOCK I/O
Single-Hart Event Loop Cpu & Devices src/runtime/event_loop.cpp EventLoop: Instruction step, interrupt checks, & device ticks

6. Summary

This line-by-line analysis demonstrates how homemade-risc-v-64-vector-linux-emulator seamlessly integrates low-level RV64GCV execution, Sv39 page tables, CLINT/PLIC interrupts, and VirtIO MMIO devices to boot a standard Linux 6.x distribution into an interactive shell.