On Fri, Jul 18, 2025 at 03:25:46PM +0000, Keir Fraser wrote: > On Fri, Jul 18, 2025 at 02:53:42PM +0000, Keir Fraser wrote: > > 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@xxxxxxxxxx> > > > > --- > > > > 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(). This is good point, I agree with you. > > Also I should note that this is moot because the smp_mb() would *not* > safely replace the load-acquire. Hmm.. do you mean it can't order the write to dist->ready here while read it in aonther thread at the same time ? > > > 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 > > > > >
