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

[David Hildenbrand]

On 06.07.21 13:56, Christian Borntraeger wrote:
> On 06.07.21 13:52, Cornelia Huck wrote:
> > On Tue, Jul 06 2021, Janis Schoetterl-Glausch <scgl@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > > 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?)
>
> Not for all hardware versions, but also no indication. The architecture
> says that the HW is free to do this or not. (which makes the vsie code
> simpler).

I remember the architecture said at some point to never set undefined 
bits - and this bit is undefined on older HW generations. I might be 
wrong, though.

(I though HW learned the lesson to always use proper feature indications 
along with new features)


--
Thanks,

David / dhildenb