linux-kernel - Re: [PATCH] KVM: s390: Enable specification exception interpretation

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Date:   Wed, 07 Jul 2021 10:54:02 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "open list:KERNEL VIRTUAL MACHINE for s390 (KVM/s390)" 
        <kvm@...r.kernel.org>,
        "open list:S390" <linux-s390@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: s390: Enable specification exception interpretation

On Wed, Jul 07 2021, Christian Borntraeger <borntraeger@...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@...ibm.com>
>> 
>> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.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@...hat.com>