Re: [PATCH] KVM: s390: Enable specification exception interpretation

Cornelia Huck
2021-07-07T08:54:14Z

On Wed, Jul 07 2021, Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 06.07.21 13:47, Janis Schoetterl-Glausch wrote:
>> When this feature is enabled the hardware is free to interpret
>> specification exceptions generated by the guest, instead of causing
>> program interruption interceptions.
>>
>> This benefits (test) programs that generate a lot of specification
>> exceptions (roughly 4x increase in exceptions/sec).
>>
>> Interceptions will occur as before if ICTL_PINT is set,
>> i.e. if guest debug is enabled.
>
> I think I will add
>
> There is no indication if this feature is available or not and the hardware
> is free to interpret or not. So we can simply set this bit and if the
> hardware ignores it we fall back to intercept 8 handling.

Sounds good.

>
>
> With that
>
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>
>> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
>> ---
>> I'll additionally send kvm-unit-tests for testing this feature.
>>
>> arch/s390/include/asm/kvm_host.h | 1 +
>> arch/s390/kvm/kvm-s390.c | 2 ++
>> arch/s390/kvm/vsie.c | 2 ++
>> 3 files changed, 5 insertions(+)

(...)

>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index b655a7d82bf0..aadd589a3755 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -3200,6 +3200,8 @@ static int kvm_s390_vcpu_setup(struct kvm_vcpu *vcpu)
>> vcpu->arch.sie_block->ecb |= ECB_SRSI;
>> if (test_kvm_facility(vcpu->kvm, 73))
>> vcpu->arch.sie_block->ecb |= ECB_TE;

Maybe add

/* no facility bit, but safe as the hardware may ignore it */

or something like that, so that we don't stumble over that in the future?

>> + if (!kvm_is_ucontrol(vcpu->kvm))
>> + vcpu->arch.sie_block->ecb |= ECB_SPECI;
>>
>> if (test_kvm_facility(vcpu->kvm, 8) && vcpu->kvm->arch.use_pfmfi)
>> vcpu->arch.sie_block->ecb2 |= ECB2_PFMFI;

Reviewed-by: Cornelia Huck <cohuck@redhat.com>