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

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Date:   Wed, 7 Jul 2021 11:55:18 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 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 */

?

> 
>>> +	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>
>