v0.5.0: SBI Firmware & Linux Boot Infrastructure

🚀 v0.5.0 — SBI Firmware & Linux Boot Infrastructure

Major milestone: microvm now has an SBI firmware layer, which is the interface Linux uses to communicate with machine-mode firmware. This is the critical missing piece for Linux kernel boot.

New Features

SBI (Supervisor Binary Interface)

• Legacy extensions: set_timer, console_putchar, console_getchar, shutdown
• SBI v0.2 Base extension: spec version, implementation ID, extension probing
• Timer extension (TIME): set_timer with proper STIP management
• IPI extension (sPI): send_ipi for single-hart system
• HSM extension: hart status reporting
• SRST extension: system reset/shutdown

PMP (Physical Memory Protection)

• pmpcfg0/pmpcfg2 and pmpaddr0-15 CSR support (RV64 layout)
• Linux probes PMP registers during early boot — no longer causes illegal instruction

Interrupt Delegation (Fixed)

• Properly checks SIE for S-mode delegated interrupts
• Correct priority handling across M/S-mode interrupt sources
• Timer interrupts now set both MTIP and STIP for S-mode delegation
• SEIP properly cleared when PLIC has no pending interrupt

CSR Improvements

• TIME CSR now reads CLINT mtime (was returning 0)
• MSTATUS initializes UXL/SXL to 2 (64-bit), preserved on writes
• Added MVENDORID, MARCHID, MIMPID read-only CSRs

Test Suite

• 68 tests total (52 integration + 16 unit tests)
• 12 new tests covering SBI calls, PMP, MSTATUS, TIME CSR, SRET, interrupt delegation

Boot Progress

With SBI firmware in place, the emulator can now handle the fundamental Linux kernel boot sequence:

1. ✅ Boot ROM → kernel entry with DTB
2. ✅ Kernel probes SBI extensions
3. ✅ Kernel sets up S-mode trap handlers
4. ✅ Timer interrupts delegate to S-mode via SBI
5. 🔜 Next: test with actual Linux kernel image