[PATCH v2 1/2] KVM: selftests: Add a helper to read a vCPU's APIC ID

[Hemanth Selam <hemanth.selam@gmail.com>]

Reading a vCPU's APIC ID from the host means open coding KVM_GET_LAPIC and
picking the field out of the register block, which several tests already
do.  Add a helper next to the other APIC definitions so that a test that
needs to target a vCPU, e.g. to send it an IPI, can just ask for its ID.

Assisted-by: Cursor:claude-opus-5
Signed-off-by: Hemanth Selam <hemanth.selam@gmail.com>
---
 tools/testing/selftests/kvm/include/x86/apic.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/testing/selftests/kvm/include/x86/apic.h b/tools/testing/selftests/kvm/include/x86/apic.h
index 31887bdc3d6c..293044c81228 100644
--- a/tools/testing/selftests/kvm/include/x86/apic.h
+++ b/tools/testing/selftests/kvm/include/x86/apic.h
@@ -79,6 +79,15 @@ void apic_disable(void);
 void xapic_enable(void);
 void x2apic_enable(void);
 
+/* Reads the APIC ID of a vCPU from the host, e.g. to target an IPI at it. */
+static inline u32 vcpu_get_apic_id(struct kvm_vcpu *vcpu)
+{
+	struct kvm_lapic_state lapic;
+
+	vcpu_ioctl(vcpu, KVM_GET_LAPIC, &lapic);
+	return GET_APIC_ID_FIELD(*(u32 *)&lapic.regs[APIC_ID]);
+}
+
 static inline u32 get_bsp_flag(void)
 {
 	return rdmsr(MSR_IA32_APICBASE) & MSR_IA32_APICBASE_BSP;
-- 
2.43.7