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

[David Hildenbrand]

On 06.07.21 17:27, Janis Schoetterl-Glausch wrote:
> On 7/6/21 5:16 PM, David Hildenbrand wrote:
> > On 06.07.21 14:02, Christian Borntraeger wrote:
> > > On 06.07.21 13:59, David Hildenbrand wrote:
> > > > 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 can confirm that this bit will be ignored on older machines. The notion of
> > > never setting undefined bits comes from "you never know what this bit will
> > > change in future machines". Now we know :-)
> >
> > Well, okay then :)
> >
> > So the plan for vSIE is to always keep it disabled? IIUC, one could similarly always forward the bit of set.
>
> The bit does get copied for vSIE.

... and I missed that hunk :)

LGTM then

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>


--
Thanks,

David / dhildenb