CLUI

CLUI clears the user-interrupt flag, UIF, and holds back user interrupts from the next instruction boundary on: unlike CLI, it needs no privilege, so a program can protect its own critical sections from its own user-interrupt handler, the way a kernel does with IF. STUI sets the flag again and TESTUI reads it.

What a held interrupt does#

CLUI does not discard anything. A user interrupt that arrives while UIF is clear stays recorded as a request, and requests posted for the thread by SENDUIPI keep accumulating, one bit per vector; once UIF is set again, the processor delivers the highest pending vector at the next instruction boundary. Several requests with the same vector merge into one bit, so a handler cannot count deliveries to count events.

Delivery itself clears UIF before the handler’s first instruction, so a handler does not need a CLUI to keep from being interrupted by the next user interrupt.

In compiled code#

GCC 14 and Clang 23 with -muintr at -O2 compile _clui() to clui. A section that must work whether or not user interrupts were enabled on entry reads the flag with _testui() first and sets it again 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#

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

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 = 0                          # at once: no user interrupt is
                                 # delivered after it

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 CLUI, 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

• SENDUIPI
• STUI
• TESTUI
• UIRET