[PATCH v2 2/4] KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering

[Sean Christopherson]

On Thu, Jul 17, 2025, Yao Yuan wrote:
> On Wed, Jul 16, 2025 at 11:07:35AM +0800, Keir Fraser wrote:
> > In preparation to remove synchronize_srcu() from MMIO registration,
> > remove the distributor's dependency on this implicit barrier by
> > direct acquire-release synchronization on the flag write and its
> > lock-free check.
> >
> > Signed-off-by: Keir Fraser <keirf at google.com>
> > ---
> >  arch/arm64/kvm/vgic/vgic-init.c | 11 ++---------
> >  1 file changed, 2 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> > index 502b65049703..bc83672e461b 100644
> > --- a/arch/arm64/kvm/vgic/vgic-init.c
> > +++ b/arch/arm64/kvm/vgic/vgic-init.c
> > @@ -567,7 +567,7 @@ int kvm_vgic_map_resources(struct kvm *kvm)
> >  	gpa_t dist_base;
> >  	int ret = 0;
> >
> > -	if (likely(dist->ready))
> > +	if (likely(smp_load_acquire(&dist->ready)))
> >  		return 0;
> >
> >  	mutex_lock(&kvm->slots_lock);
> > @@ -598,14 +598,7 @@ int kvm_vgic_map_resources(struct kvm *kvm)
> >  		goto out_slots;
> >  	}
> >
> > -	/*
> > -	 * kvm_io_bus_register_dev() guarantees all readers see the new MMIO
> > -	 * registration before returning through synchronize_srcu(), which also
> > -	 * implies a full memory barrier. As such, marking the distributor as
> > -	 * 'ready' here is guaranteed to be ordered after all vCPUs having seen
> > -	 * a completely configured distributor.
> > -	 */
> > -	dist->ready = true;
> > +	smp_store_release(&dist->ready, true);
> 
> No need the store-release and load-acquire for replacing
> synchronize_srcu_expedited() w/ call_srcu() IIUC:

This isn't about using call_srcu(), because it's not actually about kvm->buses.
This code is concerned with ensuring that all stores to kvm->arch.vgic are ordered
before the store to set kvm->arch.vgic.ready, so that vCPUs never see "ready==true"
with a half-baked distributor.

In the current code, kvm_vgic_map_resources() relies on the synchronize_srcu() in
kvm_io_bus_register_dev() to provide the ordering guarantees.  Switching to
smp_store_release() + smp_load_acquire() removes the dependency on the
synchronize_srcu() so that the synchronize_srcu() call can be safely removed.