linux-kernel - Re: [PATCH v2] KVM: s390: Enable specification exception interpretation

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Date:   Wed, 7 Jul 2021 14:59:13 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     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>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: s390: Enable specification exception
 interpretation



On 07.07.21 14:07, 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, but it is not required to.
> There is no indication if this feature is available or not,
> so we can simply set this bit and if the hardware ignores it
> we fall back to intercept 8 handling.
> The same applies to vSIE, we forward the guest hypervisor's bit
> and fall back to injection if interpretation does not occur.
> 
> 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.
> A specification exception detected on the program new PSW
> will also always cause an interception.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>
> Reviewed-by: Cornelia Huck <cohuck@...hat.com>

Thanks, applied.

> ---
> v1 -> v2
> 	Rephrase commit message
> 	Add comment about opting into interpretation
> 
>   arch/s390/include/asm/kvm_host.h | 1 +
>   arch/s390/kvm/kvm-s390.c         | 5 +++++
>   arch/s390/kvm/vsie.c             | 2 ++
>   3 files changed, 8 insertions(+)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 9b4473f76e56..3a5b5084cdbe 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -244,6 +244,7 @@ struct kvm_s390_sie_block {
>   	__u8	fpf;			/* 0x0060 */
>   #define ECB_GS		0x40
>   #define ECB_TE		0x10
> +#define ECB_SPECI	0x08
>   #define ECB_SRSI	0x04
>   #define ECB_HOSTPROTINT	0x02
>   	__u8	ecb;			/* 0x0061 */
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index b655a7d82bf0..7675b72a3ddf 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3200,6 +3200,11 @@ 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;
> +	/* 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;
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 4002a24bc43a..acda4b6fc851 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -510,6 +510,8 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   			prefix_unmapped(vsie_page);
>   		scb_s->ecb |= ECB_TE;
>   	}
> +	/* specification exception interpretation */
> +	scb_s->ecb |= scb_o->ecb & ECB_SPECI;
>   	/* branch prediction */
>   	if (test_kvm_facility(vcpu->kvm, 82))
>   		scb_s->fpf |= scb_o->fpf & FPF_BPBC;
>