TESTUI

TESTUI reads the user-interrupt flag, UIF, into CF: CF = 1 means that user interrupts can be delivered, CF = 0 that they are held back, by CLUI or because the code is a user-interrupt handler, whose delivery cleared the flag. It is how a program reads UIF directly; STUI and CLUI write it. Like them, it runs at any privilege level, but unlike them it does not abort a TSX transaction.

The other flags#

Intel’s manual clears ZF, OF, AF, PF and SF, so after TESTUI the flags hold UIF and nothing else. XED, from which this page takes its flags table, records only CF, as the note beside the table says; code should not rely on the other five keeping their values across a TESTUI.

In compiled code#

_testui() returns UIF as an unsigned char. With -muintr, GCC 14 and Clang 23 at -O2 compile return _testui(); to xor eax, eax, TESTUI and setc al (Clang writes setb). Its main use is to save the flag before a critical section and restore it afterwards: both compilers turned

unsigned char was = _testui();
_clui();
/* ... */
if (was)
    _stui();

into TESTUI, a SETcc of CF, CLUI, the protected code and a branch around the STUI, so that a section entered with user interrupts held back leaves them held back.

Where it runs#

TESTUI exists only in 64-bit mode and needs UINTR (CPUID leaf 7, EDX bit 5) and CR4.UINTR; otherwise, inside an enclave or with a LOCK prefix, it raises #UD. This site’s Zen 5 reports no UINTR, and both recorded cases raised #UD and left RFLAGS as it was, CF included.

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)
cf = uif
zf = af = of = pf = sf = 0

Operands

Flags

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
• STUI
• UIRET