KVM: selftests: Add helpers for locally (un)blocking IRQs on x86

Copy KVM-Unit-Tests' x86 helpers for emitting STI and CLI, comments and
all, and use them throughout x86 selftests.  The safe_halt() and sti_nop()
logic in particular benefits from centralized comments, as the behavior
isn't obvious unless the reader is already aware of the STI shadow.

Cc: Manali Shukla <Manali.Shukla@amd.com>
Link: https://lore.kernel.org/r/20241220012617.3513898-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/x86/processor.h

@@ -1335,6 +1335,46 @@ static inline void kvm_hypercall_map_gpa_range(uint64_t gpa, uint64_t size,
 	GUEST_ASSERT(!ret);
 }
 
+/*
+ * Execute HLT in an STI interrupt shadow to ensure that a pending IRQ that's
+ * intended to be a wake event arrives *after* HLT is executed.  Modern CPUs,
+ * except for a few oddballs that KVM is unlikely to run on, block IRQs for one
+ * instruction after STI, *if* RFLAGS.IF=0 before STI.  Note, Intel CPUs may
+ * block other events beyond regular IRQs, e.g. may block NMIs and SMIs too.
+ */
+static inline void safe_halt(void)
+{
+	asm volatile("sti; hlt");
+}
+
+/*
+ * Enable interrupts and ensure that interrupts are evaluated upon return from
+ * this function, i.e. execute a nop to consume the STi interrupt shadow.
+ */
+static inline void sti_nop(void)
+{
+	asm volatile ("sti; nop");
+}
+
+/*
+ * Enable interrupts for one instruction (nop), to allow the CPU to process all
+ * interrupts that are already pending.
+ */
+static inline void sti_nop_cli(void)
+{
+	asm volatile ("sti; nop; cli");
+}
+
+static inline void sti(void)
+{
+	asm volatile("sti");
+}
+
+static inline void cli(void)
+{
+	asm volatile ("cli");
+}
+
 void __vm_xsave_require_permission(uint64_t xfeature, const char *name);
 
 #define vm_xsave_require_permission(xfeature)	\

tools/testing/selftests/kvm/x86/hyperv_ipi.c

@@ -63,8 +63,10 @@ static void receiver_code(void *hcall_page, vm_vaddr_t pgs_gpa)
 	/* Signal sender vCPU we're ready */
 	ipis_rcvd[vcpu_id] = (u64)-1;
 
-	for (;;)
-		asm volatile("sti; hlt; cli");
+	for (;;) {
+		safe_halt();
+		cli();
+	}
 }
 
 static void guest_ipi_handler(struct ex_regs *regs)

tools/testing/selftests/kvm/x86/svm_int_ctl_test.c

@@ -42,10 +42,7 @@ static void l2_guest_code(struct svm_test_data *svm)
 	x2apic_write_reg(APIC_ICR,
 		APIC_DEST_SELF | APIC_INT_ASSERT | INTR_IRQ_NUMBER);
 
-	__asm__ __volatile__(
-		"sti\n"
-		"nop\n"
-	);
+	sti_nop();
 
 	GUEST_ASSERT(vintr_irq_called);
 	GUEST_ASSERT(intr_irq_called);

tools/testing/selftests/kvm/x86/ucna_injection_test.c

@@ -86,7 +86,7 @@ static void ucna_injection_guest_code(void)
 	wrmsr(MSR_IA32_MCx_CTL2(UCNA_BANK), ctl2 | MCI_CTL2_CMCI_EN);
 
 	/* Enables interrupt in guest. */
-	asm volatile("sti");
+	sti();
 
 	/* Let user space inject the first UCNA */
 	GUEST_SYNC(SYNC_FIRST_UCNA);

tools/testing/selftests/kvm/x86/xapic_ipi_test.c

@@ -106,7 +106,8 @@ static void halter_guest_code(struct test_data_page *data)
 		data->halter_tpr = xapic_read_reg(APIC_TASKPRI);
 		data->halter_ppr = xapic_read_reg(APIC_PROCPRI);
 		data->hlt_count++;
-		asm volatile("sti; hlt; cli");
+		safe_halt();
+		cli();
 		data->wake_count++;
 	}
 }

tools/testing/selftests/kvm/x86/xapic_state_test.c

@@ -18,7 +18,7 @@ struct xapic_vcpu {
 
 static void xapic_guest_code(void)
 {
-	asm volatile("cli");
+	cli();
 
 	xapic_enable();
 
@@ -38,7 +38,7 @@ static void xapic_guest_code(void)
 
 static void x2apic_guest_code(void)
 {
-	asm volatile("cli");
+	cli();
 
 	x2apic_enable();
 

tools/testing/selftests/kvm/x86/xen_shinfo_test.c

@@ -191,10 +191,7 @@ static void guest_code(void)
 	struct vcpu_runstate_info *rs = (void *)RUNSTATE_VADDR;
 	int i;
 
-	__asm__ __volatile__(
-		"sti\n"
-		"nop\n"
-	);
+	sti_nop();
 
 	/* Trigger an interrupt injection */
 	GUEST_SYNC(TEST_INJECT_VECTOR);