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

[Keir Fraser <keirf@google.com>]

On Thu, Jul 17, 2025 at 01:44:48PM +0800, 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@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:
> 
> Tree SRCU on SMP:
> call_srcu()
>  __call_srcu()
>   srcu_gp_start_if_needed()
>     __srcu_read_unlock_nmisafe()
> 	 #ifdef	CONFIG_NEED_SRCU_NMI_SAFE
> 	   	  smp_mb__before_atomic() // __smp_mb() on ARM64, do nothing on x86.
> 	 #else
>           __srcu_read_unlock()
> 		   smp_mb()
> 	 #endif

I don't think it's nice to depend on an implementation detail of
kvm_io_bus_register_dev() and, transitively, on implementation details
of call_srcu().

kvm_vgic_map_resources() isn't called that often and can afford its
own synchronization.

 -- Keir

> TINY SRCY on UP:
> Should have no memory ordering issue on UP.
> 
> >  	goto out_slots;
> >  out:
> >  	mutex_unlock(&kvm->arch.config_lock);
> > --
> > 2.50.0.727.gbf7dc18ff4-goog
> >