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

[Michael Mueller]

On 04.01.19 14:19, Cornelia Huck wrote:
> On Wed, 2 Jan 2019 18:29:00 +0100
> Pierre Morel <pmorel@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > >    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.

There is a rc but a check here is not required.

There are three cases:

a) This is the first ISC that gets registered, then the GISA is
   not in use and IAM is set in the GISA.

b) A second ISC gets registered and the GISA is *not* in the
   alert list. Then the IAM is set here as well.

c) A second ISC gets registered and the GISA is in the
   alert list. Then the IAM is intentionally not set here
   by set_iam(). It will be restored by get_ipm() with
   the new IAM value by the gib alert processing code.


> 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?

It is the same here.

> > > +	}
> > > +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)
> > >