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Diffstat (limited to 'Documentation/arch/x86/x86_64')
| -rw-r--r-- | Documentation/arch/x86/x86_64/fred.rst | 96 | ||||
| -rw-r--r-- | Documentation/arch/x86/x86_64/index.rst | 1 |
2 files changed, 97 insertions, 0 deletions
diff --git a/Documentation/arch/x86/x86_64/fred.rst b/Documentation/arch/x86/x86_64/fred.rst new file mode 100644 index 000000000000..9f57e7b91f7e --- /dev/null +++ b/Documentation/arch/x86/x86_64/fred.rst @@ -0,0 +1,96 @@ +.. 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. diff --git a/Documentation/arch/x86/x86_64/index.rst b/Documentation/arch/x86/x86_64/index.rst index a56070fc8e77..ad15e9bd623f 100644 --- a/Documentation/arch/x86/x86_64/index.rst +++ b/Documentation/arch/x86/x86_64/index.rst @@ -15,3 +15,4 @@ x86_64 Support cpu-hotplug-spec machinecheck fsgs + fred |