Re: [PATCH v12 06/37] Documentation/x86/64: Add a documentation for FRED

[Bagas Sanjaya]

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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