[PATCH v2] KVM: s390: Enable specification exception interpretation

[PATCH v2] KVM: s390: Enable specification exception interpretation

[Janis Schoetterl-Glausch]

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@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
---
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;
-- 
2.25.1

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

[Christian Borntraeger]

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@linux.ibm.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.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;
>