97 lines
4.0 KiB
ReStructuredText
97 lines
4.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
||
|
||
=========================================
|
||
Flexible Return and Event Delivery (FRED)
|
||
=========================================
|
||
|
||
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.
|
||
|
||
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.
|
||
|
||
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.
|
||
|
||
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).
|
||
|
||
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.
|