Re: [PATCH] KVM: s390: Enable specification exception interpretation

[David Hildenbrand <david@redhat.com>]

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.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.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/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..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;
>   
>   	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;
> 

I assume this is a new CPU feature, right? If so

a) How can we check whether we can actually safely enable it. (which 
facility do we have to check)
b) Do we have to handle vSIE? Do we have to indicate a CPU feature that 
unlocks this feature?

-- 
Thanks,

David / dhildenb