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

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Date:   Wed, 07 Jul 2021 13:42:23 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com>,
        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, Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com> wrote:

> On 7/7/21 10:54 AM, Cornelia Huck wrote:
>
> [...]
>
>> 
>>>> 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?
>
> Well, the hardware being allowed to ignore the bit makes its introduction
> without an indication forward compatible because it does not require vSIE to be adapted.
> The reserved bits are implicitly set to 0 which means new features are disabled
> by default and one observes all the interception one expects.
>
> Maybe this:
>
> /* no facility bit, can opt in because we do not need
>    to observe specification exception intercepts */
>
> ?

Works for me as well.

>
>> 
>>>> +	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>
>>