29.9. Flexible Return and Event Delivery (FRED)

29.9.1. Overview

The FRED architecture defines simple new transitions that change privilege level (ring transitions). The FRED architecture was designed with the following goals:

  1. Improve overall performance and response time by replacing event delivery through the interrupt descriptor table (IDT event delivery) and event return by the IRET instruction with lower latency transitions.

  2. Improve software robustness by ensuring that event delivery establishes the full supervisor context and that event return establishes the full user context.

The new transitions defined by the FRED architecture are FRED event delivery and, for returning from events, two FRED return instructions. FRED event delivery can effect a transition from ring 3 to ring 0, but it is used also to deliver events incident to ring 0. One FRED instruction (ERETU) effects a return from ring 0 to ring 3, while the other (ERETS) returns while remaining in ring 0. Collectively, FRED event delivery and the FRED return instructions are FRED transitions.

In addition to these transitions, the FRED architecture defines a new instruction (LKGS) for managing the state of the GS segment register. The LKGS instruction can be used by 64-bit operating systems that do not use the new FRED transitions.

Furthermore, the FRED architecture is easy to extend for future CPU architectures.

29.9.2. Software based event dispatching

FRED operates differently from IDT in terms of event handling. Instead of directly dispatching an event to its handler based on the event vector, FRED requires the software to dispatch an event to its handler based on both the event's type and vector. Therefore, an event dispatch framework must be implemented to facilitate the event-to-handler dispatch process. The FRED event dispatch framework takes control once an event is delivered, and employs a two-level dispatch.

The first level dispatching is event type based, and the second level dispatching is event vector based.

29.9.3. Full supervisor/user context

FRED event delivery atomically save and restore full supervisor/user context upon event delivery and return. Thus it avoids the problem of transient states due to %cr2 and/or %dr6, and it is no longer needed to handle all the ugly corner cases caused by half baked entry states.

FRED allows explicit unblock of NMI with new event return instructions ERETS/ERETU, avoiding the mess caused by IRET which unconditionally unblocks NMI, e.g., when an exception happens during NMI handling.

FRED always restores the full value of %rsp, thus ESPFIX is no longer needed when FRED is enabled.

29.9.4. LKGS

LKGS behaves like the MOV to GS instruction except that it loads the base address into the IA32_KERNEL_GS_BASE MSR instead of the GS segment’s descriptor cache. With LKGS, it ends up with avoiding mucking with kernel GS, i.e., an operating system can always operate with its own GS base address.

Because FRED event delivery from ring 3 and ERETU both swap the value of the GS base address and that of the IA32_KERNEL_GS_BASE MSR, plus the introduction of LKGS instruction, the SWAPGS instruction is no longer needed when FRED is enabled, thus is disallowed (#UD).

29.9.5. Stack levels

4 stack levels 0~3 are introduced to replace the nonreentrant IST for event handling, and each stack level should be configured to use a dedicated stack.

The current stack level could be unchanged or go higher upon FRED event delivery. If unchanged, the CPU keeps using the current event stack. If higher, the CPU switches to a new event stack specified by the MSR of the new stack level, i.e., MSR_IA32_FRED_RSP[123].

Only execution of a FRED return instruction ERET[US], could lower the current stack level, causing the CPU to switch back to the stack it was on before a previous event delivery that promoted the stack level.