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

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Date:   Tue, 06 Jul 2021 13:52:02 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        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 Tue, Jul 06 2021, Janis Schoetterl-Glausch <scgl@...ux.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@...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;
> +	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?)

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