Hypervisor development for FreeBSD
Bringing RISC-V architecture support
RISC-V is one of the most interesting architectures right now for hypervisor work — not because it's "ready", but because a lot of the stack is still being defined. Good news that RISC-V hypervisor spec is ratified and wont change anymore. Lets take a look on it together. If you're coming from x86 (KVM/VMX/SVM) or ARM (KVM/EL2), the mental model is familiar — but the details are much more fluid.
What is Bhyve?
Bhyve (pronounced "bee hive") is a modern, lightweight Type-2 hypervisor and virtual machine manager primarily designed for FreeBSD, allowing users to run guest operating systems like Linux, Windows, and other BSDs.
RISC-V and Hypervisors: The Big Picture
At a high level, RISC-V virtualization is built around a privilege model extended to support hypervisors:
- M-mode (Machine) – firmware / SBI
- HS-mode (Hypervisor Supervisor) – the host kernel (FreeBSD with Bhyve)
- VS-mode (Virtual Supervisor) – guest kernel
- VU-mode (Virtual User) – guest userspace
This is defined by the RISC-V Hypervisor Extension, often just called the H-extension.
If you've worked with bhyve before, HS-mode is effectively where virtual machine monitor lives, and VS-mode is your guest kernel context.
Key Architectural Pieces
1. Two-Stage Address Translation
RISC-V uses a two-stage MMU model:
- VS-stage: Guest virtual → guest physical
- G-stage: Guest physical → host physical
The hardware uses page table formats like:
- Sv39 / Sv48 (guest)
- G-stage equivalents (e.g., Sv39x4)
2. Trap and Emulation Model
On exception, guest traps into HS-mode, then hypervisor has to decide:
- Emulate instruction/device
- Delegate fault back to guest
- Inject virtual interrupts
3. CSRs (Control and Status Registers)
Virtualization adds a set of hypervisor CSRs:
- hstatus, hedeleg, hideleg
- hgatp (address pointer to a MMU directory for the guest)
These control:
- What traps go to HS vs VS
- Guest execution state
- Address translation context
4. SBI Layer
RISC-V systems rely on the Supervisor Binary Interface, typically implemented by firmware like OpenSBI.
For virtualization, host kernel (HS) fully emulates SBI for the guest.