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

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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




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