On Mon, Oct 02, 2023 at 11:24:27PM -0700, Xin Li wrote: > 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. LGTM, thanks! Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
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