Re: [PATCH] KVM: s390: Add capability that forwards operation exceptions

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Wed, 29 Oct 2025 13:04:11 +0000
Janosch Frank <frankja@linux.ibm.com> wrote:

> 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>

> ---
> 
> This is based on the api documentation ordering fix that's in our next
> branch.
> 
> ---
>  Documentation/virt/kvm/api.rst                |  17 ++-
>  arch/s390/include/asm/kvm_host.h              |   1 +
>  arch/s390/kvm/intercept.c                     |   3 +
>  arch/s390/kvm/kvm-s390.c                      |   7 +
>  include/uapi/linux/kvm.h                      |   1 +
>  tools/testing/selftests/kvm/Makefile.kvm      |   1 +
>  .../selftests/kvm/s390/user_operexec.c        | 140 ++++++++++++++++++
>  7 files changed, 169 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/kvm/s390/user_operexec.c
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 72b2fae99a83..67837207dc9b 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7820,7 +7820,7 @@ where 0xff represents CPUs 0-7 in cluster 0.
>  :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
> @@ -8703,6 +8703,21 @@ 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 do not (yet)
> +have an opcode.
> +
> +This capability can be enabled dynamically even if VCPUs were already
> +created and are running.
> +
>  8. Other capabilities.
>  ======================
>  
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 22cedcaea475..1e4829c70216 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -648,6 +648,7 @@ struct kvm_arch {
>  	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;
> diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c
> index c7908950c1f4..420ae62977e2 100644
> --- a/arch/s390/kvm/intercept.c
> +++ b/arch/s390/kvm/intercept.c
> @@ -471,6 +471,9 @@ static int handle_operexc(struct kvm_vcpu *vcpu)
>  	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));
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 70ebc54b1bb1..56d4730b7c41 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -606,6 +606,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>  	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:
> @@ -921,6 +922,12 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
>  		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;
>  		break;
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 52f6000ab020..8ab07396ce3b 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -963,6 +963,7 @@ struct kvm_enable_cap {
>  #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;
> diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
> index 148d427ff24b..87e429206bb8 100644
> --- a/tools/testing/selftests/kvm/Makefile.kvm
> +++ b/tools/testing/selftests/kvm/Makefile.kvm
> @@ -194,6 +194,7 @@ TEST_GEN_PROGS_s390 += s390/debug_test
>  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)
> diff --git a/tools/testing/selftests/kvm/s390/user_operexec.c b/tools/testing/selftests/kvm/s390/user_operexec.c
> new file mode 100644
> index 000000000000..714906c1d12a
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/s390/user_operexec.c
> @@ -0,0 +1,140 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Test operation exception forwarding.
> + *
> + * Copyright IBM Corp. 2025
> + *
> + * Authors:
> + *  Janosch Frank <frankja@linux.ibm.com>
> + */
> +#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();
> +}