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

Christian Borntraeger
2021-07-07T08:31:05Z

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.

I think I will add

There is no indication if this feature is available or not and the hardware
is free to interpret or not. So we can simply set this bit and if the
hardware ignores it we fall back to intercept 8 handling.

With that

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
>
> 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;
>