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

David Hildenbrand
2021-07-06T12:03:48Z

On 06.07.21 13:56, Christian Borntraeger wrote:
>
>
> On 06.07.21 13:52, Cornelia Huck wrote:
>> On Tue, Jul 06 2021, Janis Schoetterl-Glausch <scgl@linux.ibm.com> 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.
>>>
>>> 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;
>>> + if (!kvm_is_ucontrol(vcpu->kvm))
>>> + vcpu->arch.sie_block->ecb |= ECB_SPECI;
>>
>> Does this exist for any hardware version (i.e. not guarded by a cpu
>> feature?)
>
> Not for all hardware versions, but also no indication. The architecture
> says that the HW is free to do this or not. (which makes the vsie code
> simpler).

I remember the architecture said at some point to never set undefined
bits - and this bit is undefined on older HW generations. I might be
wrong, though.

(I though HW learned the lesson to always use proper feature indications
along with new features)

--
Thanks,

David / dhildenb