97 lines
4.0 KiB
ReStructuredText
97 lines
4.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=========================================
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Flexible Return and Event Delivery (FRED)
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=========================================
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Overview
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========
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The FRED architecture defines simple new transitions that change
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privilege level (ring transitions). The FRED architecture was
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designed with the following goals:
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1) Improve overall performance and response time by replacing event
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delivery through the interrupt descriptor table (IDT event
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delivery) and event return by the IRET instruction with lower
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latency transitions.
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2) Improve software robustness by ensuring that event delivery
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establishes the full supervisor context and that event return
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establishes the full user context.
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The new transitions defined by the FRED architecture are FRED event
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delivery and, for returning from events, two FRED return instructions.
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FRED event delivery can effect a transition from ring 3 to ring 0, but
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it is used also to deliver events incident to ring 0. One FRED
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instruction (ERETU) effects a return from ring 0 to ring 3, while the
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other (ERETS) returns while remaining in ring 0. Collectively, FRED
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event delivery and the FRED return instructions are FRED transitions.
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In addition to these transitions, the FRED architecture defines a new
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instruction (LKGS) for managing the state of the GS segment register.
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The LKGS instruction can be used by 64-bit operating systems that do
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not use the new FRED transitions.
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Furthermore, the FRED architecture is easy to extend for future CPU
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architectures.
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Software based event dispatching
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================================
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FRED operates differently from IDT in terms of event handling. Instead
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of directly dispatching an event to its handler based on the event
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vector, FRED requires the software to dispatch an event to its handler
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based on both the event's type and vector. Therefore, an event dispatch
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framework must be implemented to facilitate the event-to-handler
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dispatch process. The FRED event dispatch framework takes control
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once an event is delivered, and employs a two-level dispatch.
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The first level dispatching is event type based, and the second level
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dispatching is event vector based.
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Full supervisor/user context
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============================
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FRED event delivery atomically save and restore full supervisor/user
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context upon event delivery and return. Thus it avoids the problem of
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transient states due to %cr2 and/or %dr6, and it is no longer needed
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to handle all the ugly corner cases caused by half baked entry states.
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FRED allows explicit unblock of NMI with new event return instructions
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ERETS/ERETU, avoiding the mess caused by IRET which unconditionally
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unblocks NMI, e.g., when an exception happens during NMI handling.
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FRED always restores the full value of %rsp, thus ESPFIX is no longer
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needed when FRED is enabled.
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LKGS
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====
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LKGS behaves like the MOV to GS instruction except that it loads the
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base address into the IA32_KERNEL_GS_BASE MSR instead of the GS
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segment’s descriptor cache. With LKGS, it ends up with avoiding
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mucking with kernel GS, i.e., an operating system can always operate
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with its own GS base address.
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Because FRED event delivery from ring 3 and ERETU both swap the value
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of the GS base address and that of the IA32_KERNEL_GS_BASE MSR, plus
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the introduction of LKGS instruction, the SWAPGS instruction is no
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longer needed when FRED is enabled, thus is disallowed (#UD).
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Stack levels
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============
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4 stack levels 0~3 are introduced to replace the nonreentrant IST for
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event handling, and each stack level should be configured to use a
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dedicated stack.
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The current stack level could be unchanged or go higher upon FRED
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event delivery. If unchanged, the CPU keeps using the current event
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stack. If higher, the CPU switches to a new event stack specified by
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the MSR of the new stack level, i.e., MSR_IA32_FRED_RSP[123].
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Only execution of a FRED return instruction ERET[US], could lower the
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current stack level, causing the CPU to switch back to the stack it was
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on before a previous event delivery that promoted the stack level.
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