Re: [PATCH v1 1/2] s390x: KVM: accept STSI for CPU topology information

[David Hildenbrand]

On 15.07.21 12:16, Cornelia Huck wrote:
> On Thu, Jul 15 2021, David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 15.07.21 11:30, Cornelia Huck wrote:
> > > On Thu, Jul 15 2021, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > On 14.07.21 17:25, Pierre Morel wrote:
> > > > > STSI(15.1.x) gives information on the CPU configuration topology.
> > > > > Let's accept the interception of STSI with the function code 15 and
> > > > > let the userland part of the hypervisor handle it.
> > > > >
> > > > > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > > > > ---
> > > > >     arch/s390/kvm/priv.c | 11 ++++++++++-
> > > > >     1 file changed, 10 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> > > > > index 9928f785c677..4ab5f8b7780e 100644
> > > > > --- a/arch/s390/kvm/priv.c
> > > > > +++ b/arch/s390/kvm/priv.c
> > > > > @@ -856,7 +856,7 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
> > > > >     	if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE)
> > > > >     		return kvm_s390_inject_program_int(vcpu, PGM_PRIVILEGED_OP);
> > > > >     
> > > > > -	if (fc > 3) {
> > > > > +	if (fc > 3 && fc != 15) {
> > > > >     		kvm_s390_set_psw_cc(vcpu, 3);
> > > > >     		return 0;
> > > > >     	}
> > > > > @@ -893,6 +893,15 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
> > > > >     			goto out_no_data;
> > > > >     		handle_stsi_3_2_2(vcpu, (void *) mem);
> > > > >     		break;
> > > > > +	case 15:
> > > > > +		if (sel1 != 1 || sel2 < 2 || sel2 > 6)
> > > > > +			goto out_no_data;
> > > > > +		if (vcpu->kvm->arch.user_stsi) {
> > > > > +			insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);
> > > > > +			return -EREMOTE;
> > >
> > > This bypasses the trace event further down.
> > >
> > > > > +		}
> > > > > +		kvm_s390_set_psw_cc(vcpu, 3);
> > > > > +		return 0;
> > > > >     	}
> > > > >     	if (kvm_s390_pv_cpu_is_protected(vcpu)) {
> > > > >     		memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
> > > > 3. User space awareness
> > > >
> > > > How can user space identify that we actually forward these intercepts?
> > > > How can it enable them? The old KVM_CAP_S390_USER_STSI capability
> > > > is not sufficient.
> > >
> > > Why do you think that it is not sufficient? USER_STSI basically says
> > > "you may get an exit that tells you about a buffer to fill in some more
> > > data for a stsi command, and we also tell you which call". If userspace
> > > does not know what to add for a certain call, it is free to just do
> > > nothing, and if it does not get some calls it would support, that should
> > > not be a problem, either?
> >
> > If you migrate your VM from machine a to machine b, from kernel a to
> > kernel b, and kernel b does not trigger exits to user space for fc=15,
> > how could QEMU spot and catch the different capabilities to make sure
> > the guest can continue using the feature?
>
> Wouldn't that imply that the USER_STSI feature, in the function-agnostic
> way it is documented, was broken from the start?

Likely. We should have forwarded everything to user space most probably 
and not try being smart in the kernel.

> Hm. Maybe we need some kind of facility where userspace can query the
> kernel and gets a list of the stsi subcodes it may get exits for, and
> possibly fail to start the migration. Having a new capability to be
> enabled for every new subcode feels like overkill. I don't think we can
> pass a payload ("enable these subfunctions") to a cap.

Maybe a new capability that forwards everything to user space when 
enabled, and lets user space handle errors.

Or a specialized one only to unlock fc=15.

--
Thanks,

David / dhildenb