Re: [RFC PATCH v3 2/2] KVM: s390: Extend the USER_SIGP capability

Eric Farman
2021-11-11T15:03:21Z

On Thu, 2021-11-11 at 10:15 +0100, David Hildenbrand wrote:
> On 10.11.21 21:33, Eric Farman wrote:
> > With commit 2444b352c3ac ("KVM: s390: forward most SIGP orders to
> > user
> > space") we have a capability that allows the "fast" SIGP orders (as
> > defined by the Programming Notes for the SIGNAL PROCESSOR
> > instruction in
> > the Principles of Operation) to be handled in-kernel, while all
> > others are
> > sent to userspace for processing.
> >
> > This works fine but it creates a situation when, for example, a
> > SIGP SENSE
> > might return CC1 (STATUS STORED, and status bits indicating the
> > vcpu is
> > stopped), when in actuality userspace is still processing a SIGP
> > STOP AND
> > STORE STATUS order, and the vcpu is not yet actually stopped. Thus,
> > the
> > SIGP SENSE should actually be returning CC2 (busy) instead of CC1.
> >
> > To fix this, add another CPU capability, dependent on the USER_SIGP
> > one,
> > and two associated IOCTLs. One IOCTL will be used by userspace to
> > mark a
> > vcpu "busy" processing a SIGP order, and cause concurrent orders
> > handled
> > in-kernel to be returned with CC2 (busy). Another IOCTL will be
> > used by
> > userspace to mark the SIGP "finished", and the vcpu free to process
> > additional orders.
> >
>
> This looks much cleaner to me, thanks!
>
> [...]
>
> > diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> > index c07a050d757d..54371cede485 100644
> > --- a/arch/s390/kvm/kvm-s390.h
> > +++ b/arch/s390/kvm/kvm-s390.h
> > @@ -82,6 +82,22 @@ static inline int is_vcpu_idle(struct kvm_vcpu
> > *vcpu)
> > return test_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask);
> > }
> >
> > +static inline bool kvm_s390_vcpu_is_sigp_busy(struct kvm_vcpu
> > *vcpu)
> > +{
> > + return (atomic_read(&vcpu->arch.sigp_busy) == 1);
>
> You can drop ()
>
> > +}
> > +
> > +static inline bool kvm_s390_vcpu_set_sigp_busy(struct kvm_vcpu
> > *vcpu)
> > +{
> > + /* Return zero for success, or -EBUSY if another vcpu won */
> > + return (atomic_cmpxchg(&vcpu->arch.sigp_busy, 0, 1) == 0) ? 0 :
> > -EBUSY;
>
> You can drop () as well.
>
> We might not need the -EBUSY semantics after all. User space can just
> track if it was set, because it's in charge of setting it.

Hrm, I added this to distinguish a newer kernel with an older QEMU, but
of course an older QEMU won't know the difference either. I'll
doublecheck that this is works fine in the different permutations.

>
> > +}
> > +
> > +static inline void kvm_s390_vcpu_clear_sigp_busy(struct kvm_vcpu
> > *vcpu)
> > +{
> > + atomic_set(&vcpu->arch.sigp_busy, 0);
> > +}
> > +
> > static inline int kvm_is_ucontrol(struct kvm *kvm)
> > {
> > #ifdef CONFIG_KVM_S390_UCONTROL
> > diff --git a/arch/s390/kvm/sigp.c b/arch/s390/kvm/sigp.c
> > index 5ad3fb4619f1..a37496ea6dfa 100644
> > --- a/arch/s390/kvm/sigp.c
> > +++ b/arch/s390/kvm/sigp.c
> > @@ -276,6 +276,10 @@ static int handle_sigp_dst(struct kvm_vcpu
> > *vcpu, u8 order_code,
> > if (!dst_vcpu)
> > return SIGP_CC_NOT_OPERATIONAL;
> >
> > + if (kvm_s390_vcpu_is_sigp_busy(dst_vcpu)) {
> > + return SIGP_CC_BUSY;
> > + }
>
> You can drop {}

Arg, I had some debug in there which needed the braces, and of course
it's unnecessary now. Thanks.

>
> > +
> > switch (order_code) {
> > case SIGP_SENSE:
> > vcpu->stat.instruction_sigp_sense++;
> > @@ -411,6 +415,12 @@ int kvm_s390_handle_sigp(struct kvm_vcpu
> > *vcpu)
> > if (handle_sigp_order_in_user_space(vcpu, order_code,
> > cpu_addr))
> > return -EOPNOTSUPP;
> >
> > + /* Check the current vcpu, if it was a target from another vcpu
> > */
> > + if (kvm_s390_vcpu_is_sigp_busy(vcpu)) {
> > + kvm_s390_set_psw_cc(vcpu, SIGP_CC_BUSY);
> > + return 0;
> > + }
>
> I don't think we need this. I think the above (checking the target of
> a
> SIGP order) is sufficient. Or which situation do you have in mind?
>

Hrm... I think you're right. I was thinking of this:

VCPU 1 - SIGP STOP CPU 2
VCPU 2 - SIGP SENSE CPU 1

But of course either CPU2 is going to be marked "busy" first, and the
sense doesn't get processed until it's reset, or the sense arrives
first, and the busy/notbusy doesn't matter. Let me doublecheck my tests
for the non-RFC version.

>
>
> I do wonder if we want to make this a kvm_arch_vcpu_ioctl() instead,

In one of my original attempts between v1 and v2, I had put this there.
This reliably deadlocks my guest, because the caller (kvm_vcpu_ioctl())
tries to acquire vcpu->mutex, and racing SIGPs (via KVM_RUN) might
already be holding it. Thus, it's an async ioctl. I could fold it into
the existing interrupt ioctl, but as those are architected structs it
seems more natural do it this way. Or I have mis-understood something
along the way?

> essentially just providing a KVM_S390_SET_SIGP_BUSY *and* providing
> the
> order. "order == 0" sets it to !busy.

I'd tried this too, since it provided some nice debug-ability.
Unfortunately, I have a testcase (which I'll eventually get folded into
kvm-unit-tests :)) that picks a random order between 0-255, knowing
that there's only a couple handfuls of valid orders, to check the
response. Zero is valid architecturally (POPS figure 4-29), even if
it's unassigned. The likelihood of it becoming assigned is probably
quite low, but I'm not sure that I like special-casing an order of zero
in this way.

> Not that we would need the value
> right now, but who knows for what we might reuse that interface in
> the
> future.
>
> Thanks!
>