STUI

STUI sets the user-interrupt flag, UIF, so that user interrupts can be delivered again; CLUI clears it and TESTUI reads it. UIF does for user interrupts what IF does for ordinary ones, but it belongs to the program: all three instructions work at CPL 3, and no operating-system setting restricts them beyond turning user interrupts on with CR4.UINTR.

No delay#

A user interrupt that is pending can be delivered right after STUI, on the very next instruction boundary. STI behaves otherwise: ordinary interrupts stay blocked until the instruction after it has completed. Code that allows user interrupts again must therefore have its state in order before the STUI itself. Intel’s manual also notes that STI’s one-instruction window does not hold back user interrupts.

Delivery clears UIF on its own, so a handler runs with user interrupts blocked; a handler can execute STUI to let another user interrupt interrupt it, and UIRET sets UIF again on the way out.

Restoring a saved state#

GCC 14 and Clang 23 with -muintr at -O2 compile _stui() to stui. For a section that must not be interrupted, the usual pattern saves the flag with _testui(), clears it with _clui() and sets it again afterwards only if it was set: both compilers made TESTUI, a SETcc of CF, CLUI, the protected code, and a test that branches around the STUI.

Where it runs#

STUI exists only in 64-bit mode, needs UINTR (CPUID leaf 7, EDX bit 5) and CR4.UINTR, and raises #UD otherwise or with a LOCK prefix. In a TSX transaction it aborts the transaction as STI would. Its bytes, F3 0F 01 EF, are those of WRPKRU with an F3 prefix; the probe does not run them, as the Operation section says, so the site has no Zen 5 result for STUI.

Forms

Latency and throughput

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

if not cr4 >> 25 & 1:            # CR4.UINTR; clear without UINTR
    fault(UD)
uif = 1                          # at once: a pending user interrupt
                                 # may be delivered right after it,
                                 # not one instruction later as an
                                 # interrupt after STI

Operands

Flags

None: XED records no flag that the forms test or change.

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to STUI, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

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.

More UINTR instructions

• CLUI
• SENDUIPI
• TESTUI
• UIRET