gdb-debugging

Testing & Quality

Guide for debugging Nanvix guests using GDB remote debugging over the microvm machine type. Use this when asked about attaching GDB, setting breakpoints, or stepping through kernel code.

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GDB Debugging Nanvix

Use this skill when the user asks about debugging the Nanvix kernel or guest applications using GDB. Refer to doc/gdb.md for the user-facing documentation.

Prerequisites

Nanvix built in standalone mode (DEPLOYMENT_MODE=standalone). The standalone feature automatically enables the gdb Cargo feature in nanvixd and nanvix-bench. At the crate level, the gdb feature requires microvm (enforced via feature dependency).
A GDB client with x86_64 target support (gdb-multiarch or x86_64-elf-gdb).

Build

./z build -- all DEPLOYMENT_MODE=standalone

Launch

Through nanvixd (preferred):

RUST_LOG=info ./bin/nanvixd.elf -gdb-port 1234 -- ./bin/hello-rust-nostd.elf

Through uservm directly:

RUST_LOG=info ./bin/uservm.elf \
    -kernel ./bin/kernel.elf \
    -initrd ./bin/hello-rust-nostd.elf \
    -standalone -gdb-port 1234

The guest halts at its first instruction until a GDB client connects.

Connect

gdb-multiarch ./bin/kernel.elf
(gdb) target remote :1234
(gdb) break _start
(gdb) continue

A ready-to-use .gdbinit exists at the repository root and is loaded automatically when GDB is launched from the project directory.

GDB Command Reference

Operation	Command	Notes
Read registers	`info registers`	All GPRs, RIP, RFLAGS, segments
Write registers	`set $rax = 0x42`	Any general-purpose register
Read memory	`x/16xb 0x1000`	Guest physical memory
Write memory	`set {int}0x1000 = 1`	Guest physical memory
Software breakpoint	`break *0x1000`	INT3-based
Remove breakpoint	`delete 1`	Restores original instruction byte
Single step	`stepi`	One instruction
Continue	`continue`	Resume until next event
Backtrace	`bt`	Requires symbols
Disassemble	`disas`	At current RIP

Common Workflows

Kernel crash

(gdb) continue
# ... guest crashes ...
(gdb) bt
(gdb) info registers
(gdb) x/10i $rip

Stepping through boot

(gdb) stepi
(gdb) info registers

Breaking on a function

(gdb) break kmain
(gdb) continue

Troubleshooting

Problem	Cause / Fix
GDB cannot connect	Port in use (`ss -tlnp \| grep 1234`), or VM not started
	with `-gdb-port`.
"Remote connection closed"	Guest exited before GDB connected. Check VM stderr.
Breakpoints not hit	Address mismatch — kernel uses identity mapping
	(GVA == GPA). Verify symbol file: `file ./bin/kernel.elf`.
Memory access returns zeros	Address outside guest memory region
	(`config::kernel::MEMORY_SIZE`).

Limitations

Linux only. GDB debugging requires the microvm machine type with KVM, which is not available on Windows. The WHP backend does not expose a GDB server.
Standalone mode only. -gdb-port requires DEPLOYMENT_MODE=standalone.
Physical addresses only. The kernel uses identity mapping (GVA == GPA).
No hardware breakpoints. Only software breakpoints (INT3).
Single vCPU. Single-threaded debugging model.
Ctrl+C latency. The break-in signal is processed at the next vCPU exit (e.g., port I/O), not asynchronously during pure guest computation. In practice this is near-instant because the Nanvix guest performs frequent I/O.

Architecture Notes

GDB server uses the gdbstub crate (v0.7), feature-gated behind gdb.
Server runs synchronously in the vCPU thread (KVM register ioctls are thread-specific).
Software breakpoints patch guest memory with INT3 (0xCC); KVM delivers VcpuExit::Debug.
The gdb feature is enabled by the standalone feature in nanvixd and nanvix-bench. At the crate level, gdb requires microvm (enforced via feature dependency). The -gdb-port flag is validated at runtime to require -standalone mode.