RISC-V Exceptions: Addressing Vsireg/sireg Access Issues
Hey everyone! Let's dive into a tricky part of the RISC-V specification, specifically how exceptions should be handled when dealing with inaccessible registers in the iregDiscussion category. This is a critical area for ensuring our simulators, like Spike, behave correctly, and it touches on the nuances between illegal instruction exceptions and virtual instruction exceptions.
Understanding the RISC-V Spec on Register Access
The RISC-V spec lays out some very specific rules about what should happen when we try to access registers that we shouldn't. Here’s the core of the issue, pulled straight from the spec:
When
vsiselecthas a reserved value (including values above0x1FFnot designated for custom use), attempts from M-mode or HS-mode to accessvsireg, or from VS-mode to accesssireg(reallyvsireg), should preferably raise an illegal instruction exception.When
vsiselecthas the number of an inaccessible register, attempts from M-mode or HS-mode to accessvsiregraise an illegal instruction exception, and attempts from VS-mode to accesssireg(reallyvsireg) raise a virtual instruction exception.
Let's break this down. The vsiselect register is used to select a specific virtual supervisor (VS) register. If vsiselect holds a reserved value—meaning it’s not assigned to a real register—or if it points to a register that the current mode doesn't have access to, things get interesting. The spec dictates that accessing vsireg or sireg in these scenarios should lead to exceptions. But there’s a key difference depending on the mode and the type of access violation.
- Reserved Values: When
vsiselectcontains a reserved value, the spec says that an illegal instruction exception should preferably be raised if the access attempt comes from M-mode (Machine mode) or HS-mode (Hypervisor-secure mode). This “preferably” is a bit of wiggle room in the spec, but it strongly suggests this is the expected behavior. For VS-mode (Virtual Supervisor mode) accessingsireg(which is reallyvsiregunder the hood in VS-mode), the same preference for an illegal instruction exception applies. - Inaccessible Register Numbers: If
vsiselectholds the number of an inaccessible register, the rules are a bit stricter. Access attempts from M-mode or HS-mode tovsiregmust raise an illegal instruction exception. However, if the access attempt is from VS-mode tosireg, a virtual instruction exception must be raised. This distinction is crucial because it differentiates between a fundamentally invalid instruction (illegal instruction) and an instruction that's valid but not allowed in the current virtualized environment (virtual instruction).
The Importance of Correct Exception Handling
Correctly handling these exceptions is crucial for several reasons:
- Security: Proper exception handling helps maintain the integrity of the system. By raising exceptions when unauthorized memory accesses occur, we prevent potentially malicious code from accessing sensitive information or corrupting the system state. This is especially vital in virtualized environments where guest operating systems must be isolated from the host and from each other.
- Debugging: Accurate exception reporting is invaluable for debugging. When an exception occurs, the system should provide enough information to pinpoint the cause, including the type of exception, the address that was accessed, and the instruction that caused the fault. This information helps developers quickly identify and fix bugs in their code.
- Compliance with Standards: Adhering to the RISC-V specification ensures that our implementations are compliant and predictable. This is essential for interoperability and for ensuring that software written for RISC-V will run correctly on different platforms. Deviations from the specification can lead to unexpected behavior and compatibility issues.
- Virtualization: In virtualized environments, the distinction between illegal instruction exceptions and virtual instruction exceptions is critical. An illegal instruction exception indicates a fundamental flaw in the instruction itself, while a virtual instruction exception signals that the instruction is valid but not allowed in the current virtualized context. This distinction allows the hypervisor to handle virtual instruction exceptions in a way that emulates the behavior of the instruction without compromising the security or stability of the host system.
Spike's Implementation: A Closer Look
Now, let’s look at how Spike, the RISC-V ISA simulator, handles these scenarios. The relevant code snippet you've pointed out is within the sscsrind_reg_csr_t::verify_permissions function:
void sscsrind_reg_csr_t::verify_permissions(insn_t insn, bool write) const {
...
csr_t_p proxy_csr = get_reg();
if (proxy_csr == nullptr) {
if (!state->v) {
throw trap_illegal_instruction(insn.bits());
} else {
throw trap_virtual_instruction(insn.bits());
}
}
proxy_csr->verify_permissions(insn, write);
}
Here’s what's happening:
get_reg(): This function tries to retrieve the CSR (Control and Status Register) proxy based on thevsiselectvalue. If the register is valid and accessible,get_reg()returns a pointer to it.proxy_csr == nullptr: Ifget_reg()can't find the expected register (becausevsiselecthas a reserved value or points to an inaccessible register), it returnsnullptr. This is where the exception handling logic kicks in.!state->v: This checks if the current mode is VS-mode.state->vis likely a boolean indicating whether the simulator is in VS-mode.throw trap_illegal_instruction(insn.bits()): If we're not in VS-mode (i.e., we're in M-mode or HS-mode), an illegal instruction exception is thrown. This aligns with the spec's requirement for M-mode and HS-mode.throw trap_virtual_instruction(insn.bits()): If we are in VS-mode, a virtual instruction exception is thrown. This also aligns with the spec for VS-mode when accessingsiregwith an inaccessiblevsiselect.
