gem5-bridge Driver for Guest-Host Simulation

• gem5-bridge driver is a Linux kernel module and userspace interface that enables non-root m5 operations by exposing a controlled API via /dev/gem5_bridge.
• It integrates into automated disk-image workflows across x86, ARM, and RISC-V, eliminating root privilege requirements and enhancing security.
• The driver standardizes guest-host communication by replacing legacy m5ops with reproducible and secure event signaling for simulation control and monitoring.

The gem5-bridge driver is a Linux kernel module and userspace interface that enables privileged in-guest operations—traditionally performed by the m5ops library—without requiring guest root access, by exposing a controlled, device-backed API through /dev/gem5_bridge. Incorporated as part of the gem5 Resources v25.0 platform, the gem5-bridge driver is a foundational mechanism for robust guest-host communication in full-system architectural simulation. Its adoption standardizes m5 operation invocation in user mode, facilitates reproducible experiment orchestration, and streamlines simulation monitoring workflows across x86, ARM, and RISC-V platforms (Pai et al., 15 Dec 2025).

1. Technical Description and Rationale

Traditional m5ops—used for orchestrating simulation exit events such as simulation termination, region-of-interest (ROI) demarcation, and checkpointing—have historically depended on mmap-ing physical addresses via /dev/mem or direct privileged accesses. This approach (1) requires root privileges within the simulated operating system, (2) is potentially insecure in multi-user workflows, and (3) complicates automation for batch simulation. The gem5-bridge driver addresses these limitations by registering a dedicated misc device, /dev/gem5_bridge, that provides a standard Unix device-node for use by userspace processes.

By integrating the driver and related m5ops binaries during automated disk-image construction (“Packer-based” workflows), gem5 Resources ensures all benchmarked workloads—regardless of privilege—can issue simulated events through calls to libm5, which is linked against workloads at build time. The driver maps a specific, per-platform MMIO region or invokes hypercall-based signaling to the simulator, with the host side handling the events as defined in the configuration (Pai et al., 15 Dec 2025).

2. Integration in Disk-Image Workflows

The gem5-bridge driver is built and installed automatically as part of the post-install provisioning phase in the Packer-based disk-image creation flow. For each ISA (x86, ARM64, RISC-V), a scripted build process compiles and loads gem5-bridge into the guest kernel, ensuring that every artifact derived from the gem5 Resources repository exposes /dev/gem5_bridge by default. As such, no manual insmod or privileged setup step is required by end-users or CI workflows.

The relevant Packer “provisioners” use shell scripts (e.g., build-gem5-bridge.sh) to handle out-of-tree kernel builds and ensure compatibility with the selected kernel version in the image (Pai et al., 15 Dec 2025).

3. User-Space Programming Interface

With gem5-bridge installed, user programs interact with the simulator through the m5ops API, which detects and opens /dev/gem5_bridge as its backend. This API remains identical to the traditional m5ops interface; only the device node, not raw physical memory, is accessed. Typical usage (C/C++/Fortran) includes:

#include <m5op.h> int main() { m5_hypercall(4); // Signal start of ROI do_workload(); m5_hypercall(5); // Signal end of ROI return 0; }

These calls perform device I/O to /dev/gem5_bridge, which the kernel module handles by writing payload values and event IDs into a buffer surfaced to the simulator process. The device supports standard open(), ioctl(), and mmap() operations as relevant to the m5ops ABI (Pai et al., 15 Dec 2025).

Prebuilt disk images in gem5 Resources (v25.0) contain both /usr/bin/m5_hypercall and the compiled driver. As a result, non-root test workloads—either launched directly in the guest shell or as part of job scripts—can invoke hypercalls, checkpoints, or simulation exits.

4. Role in Gem5 Exit Event and Hypercall System

The gem5-bridge driver is a critical element in the class-based, extensible exit event and hypercall system of gem5 v25.0. Instead of relying on order-dependent Python generators or guest-root scripts, researchers define exit events in the simulator configuration (subclassing ExitEvent, e.g., HypercallEvent). M5ops calls from the guest (delivered via gem5-bridge) carry numeric event IDs and optional payload data, which the simulator routes to the appropriate Python-side handler.

For example:

from m5.events import HypercallEvent sim.register_exit_event(HypercallEvent(4, {"name": "start-ROI"}))

In this scheme, the event demarcation, checkpointing, and ROI logic are fully decoupled from both guest OS policy and privilege, enabling fine-grained control and cross-image portability (Pai et al., 15 Dec 2025).

5. Comparison, Impact, and Best Practices

Compared to legacy /dev/mem-dependent deployments, gem5-bridge results in:

• Removal of root privilege requirements for m5ops/event functionality
• Elimination of security risk associated with exposing all physical memory to userspace in the simulation guest
• Fully automated, reproducible disk-image builds across all major ISAs without image-specific configuration
• No behavioral difference between systemd-present and systemd-free images when invoking m5ops
• Simplified workflow for parallel simulation, as each workload can trigger m5 events independently and securely

Performance studies in the reference (Pai et al., 15 Dec 2025) indicate that with gem5-bridge, booting time is reduced (systemd-free images boot up to 23× faster), and variation in region-of-interest instruction counts is contained to below 1.3 %, increasing reproducibility. All images in the v25.0 resource set are gem5-bridge enabled by default.

6. Guest–Host Coordination and Monitoring

In tandem with gem5-bridge, gem5 v25.0 introduces external signal utilities (e.g., ext_signal.py) to enable host-side statistics snapshot, simulation control, and remote event signaling. These utilities communicate with the simulator via Unix sockets, but all control and event signaling initiated from the guest continues to rely on user-mode access to /dev/gem5_bridge. No privileged daemon or auxiliary guest process is required.

Researchers configuring guest images or CI should never use /dev/mem or root-only APIs for simulation signaling under the v25.0 standard. All ROI, checkpoint, and simulation lifecycle operations should either call into /usr/bin/m5_hypercall or link against libm5, which transparently targets gem5-bridge (Pai et al., 15 Dec 2025).