Re: [PATCH v1 3/3] KVM: arm64: Using rcu_read_lock() for kvm_pgtable_stage2_mkyoung()

[Oliver Upton]

On Thu, Jun 08, 2023 at 03:05:41PM -0700, Chun-Tse Shao wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 3b9d4d24c361..0f7ea66fb894 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1437,10 +1437,10 @@ static void handle_access_fault(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
>  
>  	trace_kvm_access_fault(fault_ipa);
>  
> -	read_lock(&vcpu->kvm->mmu_lock);
> +	rcu_read_lock();
>  	mmu = vcpu->arch.hw_mmu;
>  	pte = kvm_pgtable_stage2_mkyoung(mmu->pgt, fault_ipa);
> -	read_unlock(&vcpu->kvm->mmu_lock);
> +	rcu_read_unlock();

What is the point of acquiring the RCU read lock here?
kvm_pgtable_walk_{begin,end}() already do the exact same for any
'shared' walk.

I agree with Marc that this warrants some very clear benchmark data
showing the value of the change. As I had mentioned to Yu, I already
implemented this for my own purposes, but wasn't able to see a
significant improvement over acquiring the MMU lock for read.

-- 
Thanks,
Oliver