On 14.09.23 г. 7:47 ч., Xin Li wrote:
Briefly introduce FRED, and its advantages compared to IDT. Signed-off-by: Xin Li <xin3.li@xxxxxxxxx> --- Documentation/arch/x86/x86_64/fred.rst | 98 +++++++++++++++++++++++++ Documentation/arch/x86/x86_64/index.rst | 1 + 2 files changed, 99 insertions(+) create mode 100644 Documentation/arch/x86/x86_64/fred.rst diff --git a/Documentation/arch/x86/x86_64/fred.rst b/Documentation/arch/x86/x86_64/fred.rst new file mode 100644 index 000000000000..a4ebb95f92c8 --- /dev/null +++ b/Documentation/arch/x86/x86_64/fred.rst @@ -0,0 +1,98 @@ +.. 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 swaps the value of the GS base +address and that of the IA32_KERNEL_GS_BASE MSR, and ERETU swaps 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).
nit: This will be more clear if rewritten: "Because FRED event delivery from ring 3 and ERETU both swap the value of the GS base, plus the..." .
The idea is to remove the duplicate statement that IA32_KERNEL_GS_BASE and the GS registers are swapped as it makes the sentence somewhat hard to read.
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