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

[Janosch Frank <frankja@linux.ibm.com>]

On 10/30/25 08:10, Thomas Huth wrote:
> On 29/10/2025 14.04, Janosch Frank 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>
> ...
>> +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.
> 
> "which do not (yet) have an opcode" sounds a little bit weird. Maybe rather:
> "which are not (yet) implemented in the current CPU" or so?

How about:
...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.
>>    ======================
> ...
>> 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);
> 
> Maybe check cap->flags here and return with an error if any flag is set? ...
> otherwise, if we ever add flags here, userspace cannot check whether the
> kernel accepted a flag or not.

Check the top of the function :)
I can surely add a second check to be doubly sure.

int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
{
         int r;

         if (cap->flags)
                 return -EINVAL;



>> +/*
>> + * 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();
>> +}
> 
> You could likely use the KVM_ONE_VCPU_TEST() macro and test_harness_run() to
> get rid of the boilerplate code here.

Is there a general directive to use KVM_ONE_VCPU_TEST?
To be honest I prefer the look as is since it doesn't hide things behind 
macros and 95% of our tests use it.