Re: [PATCH v5 10/15] KVM: s390: add functions to (un)register GISC with GISA

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 2 Jan 2019 18:29:00 +0100
Pierre Morel <pmorel@linux.ibm.com> wrote:

> On 19/12/2018 20:17, Michael Mueller wrote:
> > Add the IAM (Interruption Alert Mask) to the architecture specific
> > kvm struct. This mask in the GISA is used to define for which ISC
> > a GIB alert can be issued.
> > 
> > The functions kvm_s390_gisc_register() and kvm_s390_gisc_unregister()
> > are used to (un)register a GISC (guest ISC) with a virtual machine and
> > its GISA.
> > 
> > Upon successful completion, kvm_s390_gisc_register() returns the
> > ISC to be used for GIB alert interruptions. A negative return code
> > indicates an error during registration.
> > 
> > Theses functions will be used by other adapter types like AP and PCI to
> > request pass-through interruption support.
> > 
> > Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
> > ---
> >   arch/s390/include/asm/kvm_host.h |  9 ++++++
> >   arch/s390/kvm/interrupt.c        | 66 ++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 75 insertions(+)
> > 

> > +int kvm_s390_gisc_register(struct kvm *kvm, u32 gisc)
> > +{
> > +	if (!kvm->arch.gib_in_use)
> > +		return -ENODEV;
> > +	if (gisc > MAX_ISC)
> > +		return -ERANGE;
> > +
> > +	spin_lock(&kvm->arch.iam_ref_lock);
> > +	if (kvm->arch.iam_ref_count[gisc] == 0)
> > +		kvm->arch.iam |= 0x80 >> gisc;
> > +	kvm->arch.iam_ref_count[gisc]++;
> > +	if (kvm->arch.iam_ref_count[gisc] == 1)
> > +		set_iam(kvm->arch.gisa, kvm->arch.iam);  
> 
> testing the set_iam return value?
> Even it should be fine if the caller works correctly, this is done 
> before GISA is ever used.

My feeling is that checking the return code is a good idea, even if it
Should Never Fail(tm).

> 
> > +	spin_unlock(&kvm->arch.iam_ref_lock);
> > +
> > +	return gib->nisc;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_s390_gisc_register);
> > +
> > +int kvm_s390_gisc_unregister(struct kvm *kvm, u32 gisc)
> > +{
> > +	int rc = 0;
> > +
> > +	if (!kvm->arch.gib_in_use)
> > +		return -ENODEV;
> > +	if (gisc > MAX_ISC)
> > +		return -ERANGE;
> > +
> > +	spin_lock(&kvm->arch.iam_ref_lock);
> > +	if (kvm->arch.iam_ref_count[gisc] == 0) {
> > +		rc = -EINVAL;
> > +		goto out;
> > +	}
> > +	kvm->arch.iam_ref_count[gisc]--;
> > +	if (kvm->arch.iam_ref_count[gisc] == 0) {
> > +		kvm->arch.iam &= ~(0x80 >> gisc);
> > +		set_iam(kvm->arch.gisa, kvm->arch.iam);

Any chance of this function failing here? If yes, would there be any
implications?

> > +	}
> > +out:
> > +	spin_unlock(&kvm->arch.iam_ref_lock);
> > +
> > +	return rc;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_s390_gisc_unregister);
> > +
> >   void kvm_s390_gib_destroy(void)
> >   {
> >   	if (!gib)
> >   
> 
>