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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 <frankja@linux.ibm.com> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com> Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
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Lines changed: 169 additions & 1 deletion

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‎Documentation/virt/kvm/api.rst‎

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@@ -7820,7 +7820,7 @@ where 0xff represents CPUs 0-7 in cluster 0.
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:Architectures: s390
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:Parameters: none
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With this capability enabled, all illegal instructions 0x0000 (2 bytes) will
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With this capability enabled, the illegal instruction 0x0000 (2 bytes) will
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be intercepted and forwarded to user space. User space can use this
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mechanism e.g. to realize 2-byte software breakpoints. The kernel will
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not inject an operating exception for these instructions, user space has
@@ -8703,6 +8703,21 @@ This capability indicate to the userspace whether a PFNMAP memory region
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can be safely mapped as cacheable. This relies on the presence of
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force write back (FWB) feature support on the hardware.
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7.45 KVM_CAP_S390_USER_OPEREXEC
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-------------------------------
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:Architectures: s390
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:Parameters: none
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When this capability is enabled KVM forwards all operation exceptions
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that it doesn't handle itself to user space. This also includes the
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0x0000 instructions managed by KVM_CAP_S390_USER_INSTR0. This is
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helpful if user space wants to emulate instructions which are not
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(yet) implemented in hardware.
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This capability can be enabled dynamically even if VCPUs were already
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created and are running.
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8. Other capabilities.
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======================
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‎arch/s390/include/asm/kvm_host.h‎

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@@ -648,6 +648,7 @@ struct kvm_arch {
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int user_sigp;
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int user_stsi;
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int user_instr0;
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int user_operexec;
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struct s390_io_adapter *adapters[MAX_S390_IO_ADAPTERS];
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wait_queue_head_t ipte_wq;
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int ipte_lock_count;

‎arch/s390/kvm/intercept.c‎

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@@ -471,6 +471,9 @@ static int handle_operexc(struct kvm_vcpu *vcpu)
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if (vcpu->arch.sie_block->ipa == 0xb256)
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return handle_sthyi(vcpu);
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if (vcpu->kvm->arch.user_operexec)
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return -EOPNOTSUPP;
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if (vcpu->arch.sie_block->ipa == 0 && vcpu->kvm->arch.user_instr0)
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return -EOPNOTSUPP;
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rc = read_guest_lc(vcpu, __LC_PGM_NEW_PSW, &newpsw, sizeof(psw_t));

‎arch/s390/kvm/kvm-s390.c‎

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@@ -606,6 +606,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
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case KVM_CAP_SET_GUEST_DEBUG:
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case KVM_CAP_S390_DIAG318:
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case KVM_CAP_IRQFD_RESAMPLE:
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case KVM_CAP_S390_USER_OPEREXEC:
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r = 1;
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break;
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case KVM_CAP_SET_GUEST_DEBUG2:
@@ -921,6 +922,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
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VM_EVENT(kvm, 3, "ENABLE: CAP_S390_CPU_TOPOLOGY %s",
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r ? "(not available)" : "(success)");
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break;
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case KVM_CAP_S390_USER_OPEREXEC:
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VM_EVENT(kvm, 3, "%s", "ENABLE: CAP_S390_USER_OPEREXEC");
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kvm->arch.user_operexec = 1;
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icpt_operexc_on_all_vcpus(kvm);
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r = 0;
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break;
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default:
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r = -EINVAL;
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break;

‎include/uapi/linux/kvm.h‎

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@@ -963,6 +963,7 @@ struct kvm_enable_cap {
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#define KVM_CAP_RISCV_MP_STATE_RESET 242
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#define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 243
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#define KVM_CAP_GUEST_MEMFD_FLAGS 244
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#define KVM_CAP_S390_USER_OPEREXEC 245
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struct kvm_irq_routing_irqchip {
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__u32 irqchip;

‎tools/testing/selftests/kvm/Makefile.kvm‎

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@@ -194,6 +194,7 @@ TEST_GEN_PROGS_s390 += s390/debug_test
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TEST_GEN_PROGS_s390 += s390/cpumodel_subfuncs_test
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TEST_GEN_PROGS_s390 += s390/shared_zeropage_test
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TEST_GEN_PROGS_s390 += s390/ucontrol_test
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TEST_GEN_PROGS_s390 += s390/user_operexec
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TEST_GEN_PROGS_s390 += rseq_test
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TEST_GEN_PROGS_riscv = $(TEST_GEN_PROGS_COMMON)

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

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@@ -0,0 +1,140 @@
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// SPDX-License-Identifier: GPL-2.0-only
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/* Test operation exception forwarding.
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*
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* Copyright IBM Corp. 2025
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*
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* Authors:
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* Janosch Frank <frankja@linux.ibm.com>
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*/
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#include "kselftest.h"
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#include "kvm_util.h"
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#include "test_util.h"
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#include "sie.h"
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#include <linux/kvm.h>
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static void guest_code_instr0(void)
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{
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asm(".word 0x0000");
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}
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static void test_user_instr0(void)
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{
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struct kvm_vcpu *vcpu;
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struct kvm_vm *vm;
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int rc;
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vm = vm_create_with_one_vcpu(&vcpu, guest_code_instr0);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
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TEST_ASSERT_EQ(0, rc);
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vcpu_run(vcpu);
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TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0);
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kvm_vm_free(vm);
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}
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static void guest_code_user_operexec(void)
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{
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asm(".word 0x0807");
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}
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static void test_user_operexec(void)
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{
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struct kvm_vcpu *vcpu;
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struct kvm_vm *vm;
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int rc;
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vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
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TEST_ASSERT_EQ(0, rc);
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vcpu_run(vcpu);
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TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
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kvm_vm_free(vm);
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/*
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* Since user_operexec is the superset it can be used for the
63+
* 0 instruction.
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*/
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vm = vm_create_with_one_vcpu(&vcpu, guest_code_instr0);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
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TEST_ASSERT_EQ(0, rc);
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vcpu_run(vcpu);
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TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0);
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kvm_vm_free(vm);
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}
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/* combine user_instr0 and user_operexec */
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static void test_user_operexec_combined(void)
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{
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struct kvm_vcpu *vcpu;
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struct kvm_vm *vm;
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int rc;
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vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
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TEST_ASSERT_EQ(0, rc);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
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TEST_ASSERT_EQ(0, rc);
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90+
vcpu_run(vcpu);
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TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
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kvm_vm_free(vm);
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/* Reverse enablement order */
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vm = vm_create_with_one_vcpu(&vcpu, guest_code_user_operexec);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_OPEREXEC, 0);
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TEST_ASSERT_EQ(0, rc);
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rc = __vm_enable_cap(vm, KVM_CAP_S390_USER_INSTR0, 0);
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TEST_ASSERT_EQ(0, rc);
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vcpu_run(vcpu);
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TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_S390_SIEIC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.icptcode, ICPT_OPEREXC);
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TEST_ASSERT_EQ(vcpu->run->s390_sieic.ipa, 0x0807);
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109+
kvm_vm_free(vm);
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}
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/*
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* Run all tests above.
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*
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* Enablement after VCPU has been added is automatically tested since
116+
* we enable the capability after VCPU creation.
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*/
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static struct testdef {
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const char *name;
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void (*test)(void);
121+
} testlist[] = {
122+
{ "instr0", test_user_instr0 },
123+
{ "operexec", test_user_operexec },
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{ "operexec_combined", test_user_operexec_combined},
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};
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int main(int argc, char *argv[])
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{
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int idx;
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131+
TEST_REQUIRE(kvm_has_cap(KVM_CAP_S390_USER_INSTR0));
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ksft_print_header();
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ksft_set_plan(ARRAY_SIZE(testlist));
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for (idx = 0; idx < ARRAY_SIZE(testlist); idx++) {
136+
testlist[idx].test();
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ksft_test_result_pass("%s\n", testlist[idx].name);
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}
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ksft_finished();
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}

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