KVM: s390: Add capability that forwards operation exceptions

Setting KVM_CAP_S390_USER_OPEREXEC will forward all operation
exceptions to user space. This also includes the 0x0000 instructions
managed by KVM_CAP_S390_USER_INSTR0. It's helpful if user space wants
to emulate instructions which do not (yet) have an opcode.

While we're at it refine the documentation for
KVM_CAP_S390_USER_INSTR0.

Signed-off-by: Janosch Frank <[e-mail]>
Reviewed-by: Claudio Imbrenda <[e-mail]>
Acked-by: Christian Borntraeger <[e-mail]>
Signed-off-by: Janosch Frank <[e-mail]>

Documentation/virt/kvm/api.rst

···
 :Architectures: s390
 :Parameters: none
 
-With this capability enabled, all illegal instructions 0x0000 (2 bytes) will
+With this capability enabled, the illegal instruction 0x0000 (2 bytes) will
 be intercepted and forwarded to user space. User space can use this
 mechanism e.g. to realize 2-byte software breakpoints. The kernel will
 not inject an operating exception for these instructions, user space has
···
 This capability indicate to the userspace whether a PFNMAP memory region
 can be safely mapped as cacheable. This relies on the presence of
 force write back (FWB) feature support on the hardware.
+
+7.45 KVM_CAP_S390_USER_OPEREXEC
+-------------------------------
+
+:Architectures: s390
+:Parameters: none
+
+When this capability is enabled KVM forwards all operation exceptions
+that it doesn't handle itself to user space. This also includes the
+0x0000 instructions managed by KVM_CAP_S390_USER_INSTR0. This is
+helpful if user space wants to emulate instructions which are not
+(yet) implemented in hardware.
+
+This capability can be enabled dynamically even if VCPUs were already
+created and are running.
 
 8. Other capabilities.
 ======================

arch/s390/include/asm/kvm_host.h

···
 	int user_sigp;
 	int user_stsi;
 	int user_instr0;
+	int user_operexec;
 	struct s390_io_adapter *adapters[MAX_S390_IO_ADAPTERS];
 	wait_queue_head_t ipte_wq;
 	int ipte_lock_count;

arch/s390/kvm/intercept.c

···
 	if (vcpu->arch.sie_block->ipa == 0xb256)
 		return handle_sthyi(vcpu);
 
+	if (vcpu->kvm->arch.user_operexec)
+		return -EOPNOTSUPP;
+
 	if (vcpu->arch.sie_block->ipa == 0 && vcpu->kvm->arch.user_instr0)
 		return -EOPNOTSUPP;
 	rc = read_guest_lc(vcpu, __LC_PGM_NEW_PSW, &newpsw, sizeof(psw_t));

arch/s390/kvm/kvm-s390.c

···
 	case KVM_CAP_SET_GUEST_DEBUG:
 	case KVM_CAP_S390_DIAG318:
 	case KVM_CAP_IRQFD_RESAMPLE:
+	case KVM_CAP_S390_USER_OPEREXEC:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
···
 		mutex_unlock(&kvm->lock);
 		VM_EVENT(kvm, 3, "ENABLE: CAP_S390_CPU_TOPOLOGY %s",
 			 r ? "(not available)" : "(success)");
+		break;
+	case KVM_CAP_S390_USER_OPEREXEC:
+		VM_EVENT(kvm, 3, "%s", "ENABLE: CAP_S390_USER_OPEREXEC");
+		kvm->arch.user_operexec = 1;
+		icpt_operexc_on_all_vcpus(kvm);
+		r = 0;
 		break;
 	default:
 		r = -EINVAL;

include/uapi/linux/kvm.h

···
 #define KVM_CAP_RISCV_MP_STATE_RESET 242
 #define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243
 #define KVM_CAP_GUEST_MEMFD_FLAGS 244
+#define KVM_CAP_S390_USER_OPEREXEC 245
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;

tools/testing/selftests/kvm/Makefile.kvm

···
 TEST_GEN_PROGS_s390 += s390/cpumodel_subfuncs_test
 TEST_GEN_PROGS_s390 += s390/shared_zeropage_test
 TEST_GEN_PROGS_s390 += s390/ucontrol_test
+TEST_GEN_PROGS_s390 += s390/user_operexec
 TEST_GEN_PROGS_s390 += rseq_test
 
 TEST_GEN_PROGS_riscv = $(TEST_GEN_PROGS_COMMON)

tools/testing/selftests/kvm/s390/user_operexec.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+/* Test operation exception forwarding.
+ *
+ * Copyright IBM Corp. 2025
+ *
+ * Authors:
+ *  Janosch Frank <[e-mail]>
+ */
+#include "kselftest.h"
+#include "kvm_util.h"
+#include "test_util.h"
+#include "sie.h"
+
+#include <linux/kvm.h>
+
+static void guest_code_instr0(void)
+{
+	asm(".word 0x0000");
+}
+
+static void test_user_instr0(void)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	int rc;
+
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code_instr0);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
+	TEST_ASSERT_EQ(0, rc);
+
+	vcpu_run(vcpu);
+	TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0);
+
+	kvm_vm_free(vm);
+}
+
+static void guest_code_user_operexec(void)
+{
+	asm(".word 0x0807");
+}
+
+static void test_user_operexec(void)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	int rc;
+
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
+	TEST_ASSERT_EQ(0, rc);
+
+	vcpu_run(vcpu);
+	TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
+
+	kvm_vm_free(vm);
+
+	/*
+	 * Since user_operexec is the superset it can be used for the
+	 * 0 instruction.
+	 */
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code_instr0);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
+	TEST_ASSERT_EQ(0, rc);
+
+	vcpu_run(vcpu);
+	TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0);
+
+	kvm_vm_free(vm);
+}
+
+/* combine user_instr0 and user_operexec */
+static void test_user_operexec_combined(void)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	int rc;
+
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
+	TEST_ASSERT_EQ(0, rc);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
+	TEST_ASSERT_EQ(0, rc);
+
+	vcpu_run(vcpu);
+	TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
+
+	kvm_vm_free(vm);
+
+	/* Reverse enablement order */
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
+	TEST_ASSERT_EQ(0, rc);
+	rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
+	TEST_ASSERT_EQ(0, rc);
+
+	vcpu_run(vcpu);
+	TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
+	TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
+
+	kvm_vm_free(vm);
+}
+
+/*
+ * Run all tests above.
+ *
+ * Enablement after VCPU has been added is automatically tested since
+ * we enable the capability after VCPU creation.
+ */
+static struct testdef {
+	const char *name;
+	void (*test)(void);
+} testlist[] = {
+	{ "instr0", test_user_instr0 },
+	{ "operexec", test_user_operexec },
+	{ "operexec_combined", test_user_operexec_combined},
+};
+
+int main(int argc, char *argv[])
+{
+	int idx;
+
+	TEST_REQUIRE(kvm_has_cap(KVM_CAP_S390_USER_INSTR0));
+
+	ksft_print_header();
+	ksft_set_plan(ARRAY_SIZE(testlist));
+	for (idx = 0; idx < ARRAY_SIZE(testlist); idx++) {
+		testlist[idx].test();
+		ksft_test_result_pass("%s\n", testlist[idx].name);
+	}
+	ksft_finished();
+}