x86-64.net

Instruction

ERETU

Returns from a FRED event handler in ring 0 to ring 3, restoring the user context from the stack and swapping the GS base with IA32_KERNEL_GS_BASE.

Extensions
Forms
1 (1 Legacy)
Data
Intel XED v2026.08.23

ERETU is the way back to user mode when FRED is enabled. It expects RSP to point at the frame that FRED event delivery pushed, skips the error code, and restores RIP, CS, RFLAGS, RSP and SS from it, loading CS and SS for ring 3 based on IA32_STAR. It also swaps the GS base with IA32_KERNEL_GS_BASE, undoing the swap made on entry.

It works only at privilege level 0 with FRED enabled (#UD otherwise), and only at stack level 0 (#GP otherwise). With supervisor shadow stacks enabled it also checks the shadow-stack pointers. ERETS is the counterpart for events that happened in the kernel: it returns within ring 0 and leaves CS, SS and GS alone.

Forms

Every encoding of ERETU that Intel XED knows, one row per XED instruction form (iform). How to read the notation.
InstructionEncodingRequires
ERETU 64-bit mode only ERETUF3 0F 01 CAFRED

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, which covers AMD processors up to Zen 2.
ISA setCPUIDProcessors in XED
FRED

FRED
CPUID.(EAX=07H,ECX=1):EAX[17] FRED

Panther Lake, Nova Lake, Diamond Rapids

More FRED instructions

Sources

  1. Intel XED v2026.08.23 (commit 0bcb6237345c): forms, encodings, ISA sets, CPUID bits and chip model.
  2. Flexible Return and Event Delivery (FRED) Specification (346446-009)
  3. Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (325383-092, June 2026)
  4. Linux kernel documentation: Flexible Return and Event Delivery (FRED)

The tables are derived from Intel XED, Copyright Intel Corporation, licensed under the Apache License 2.0; x86-64.net converted and reformatted them. The text is our own.