linux-kernel - Re: [PATCH v2 2/4] KVM: arm64: vgic: Explicitly implement vgic

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Message-ID: <kb7nwrco6s7e6catcareyic72pxvx52jbqbfc5gbqb5zu434kg@w3rrzbut3h34>
Date: Thu, 17 Jul 2025 13:44:48 +0800
From: Yao Yuan <yaoyuan@...ux.alibaba.com>
To: Keir Fraser <keirf@...gle.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>, 
	Eric Auger <eric.auger@...hat.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 2/4] KVM: arm64: vgic: Explicitly implement
 vgic_dist::ready ordering

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@...gle.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

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
>