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...@intel.com>
  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 
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)
+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
+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 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.


Reply via email to