Re: [PATCH v6 11/13] KVM: s390: add functions to (un)register GISC with GISA

[Michael Mueller]

On 29.01.19 14:09, Cornelia Huck wrote:
> On Thu, 24 Jan 2019 13:59:37 +0100
> Michael Mueller <mimu@xxxxxxxxxxxxx> wrote:
>
> > Add the Interruption Alert Mask (IAM) to the architecture specific
> > kvm struct. This mask in the GISA is used to define for which ISC
> > a GIB alert will 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 |  13 +++++
> >   arch/s390/kvm/interrupt.c        | 117 +++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 130 insertions(+)
>
> > +int kvm_s390_gisc_register(struct kvm *kvm, u32 gisc)
> > +{
> > +	struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
> > +	u8 alert_mask;
> > +
> > +	if (!gi->origin)
> > +		return -ENODEV;
> > +	if (gisc > MAX_ISC)
> > +		return -ERANGE;
> > +
> > +	spin_lock(&gi->alert.ref_lock);
> > +	if (gi->alert.ref_count[gisc] == 0) {
>
> If the ref_count is 0 here...
>
> > +		alert_mask = gi->alert.mask | 0x80 >> gisc;
> > +		WRITE_ONCE(gi->alert.mask, alert_mask);
> > +	}
> > +	gi->alert.ref_count[gisc]++;
> > +	if (gi->alert.ref_count[gisc] == 1)
>
> ...it will certainly be 1 here, won't it?

Sure, the increment is unconditional and can be done right away.
Thus it is logically the same and now symmetric to the unregister
function:

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index ba314da746b7..f37dfb01c63c 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -2976,11 +2976,11 @@ int kvm_s390_gisc_register(struct kvm *kvm, u32 
gisc)
                return -ERANGE;

        spin_lock(&gi->alert.ref_lock);
-       if (gi->alert.ref_count[gisc] == 0)
-               gi->alert.mask |= 0x80 >> gisc;
        gi->alert.ref_count[gisc]++;
-       if (gi->alert.ref_count[gisc] == 1)
+       if (gi->alert.ref_count[gisc] == 1) {
+               gi->alert.mask |= 0x80 >> gisc;
                gisa_set_iam(gi->origin, gi->alert.mask);
+       }
        spin_unlock(&gi->alert.ref_lock);

        return gib->nisc;


> Can you do all of the manipulations in a single if branch?
>
> > +		gisa_set_iam(gi->origin, alert_mask);
> > +	spin_unlock(&gi->alert.ref_lock);
> > +
> > +	return gib->nisc;
> > +}

Thanks,
Michael