linux-kernel - Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map

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Date:   Thu, 1 Apr 2021 12:32:13 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP
 MMU map

On 02/02/21 19:57, Ben Gardon wrote:
> @@ -720,7 +790,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
>   		 */
>   		if (is_shadow_present_pte(iter.old_spte) &&
>   		    is_large_pte(iter.old_spte)) {
> -			tdp_mmu_set_spte(vcpu->kvm, &iter, 0);
> +			if (!tdp_mmu_set_spte_atomic(vcpu->kvm, &iter, 0))
> +				break;
>   
>   			kvm_flush_remote_tlbs_with_address(vcpu->kvm, iter.gfn,
>   					KVM_PAGES_PER_HPAGE(iter.level));
>
>  			/*
>  			 * The iter must explicitly re-read the spte here
>  			 * because the new value informs the !present
>                          * path below.
>                          */
>                         iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
>                 }
> 
>                 if (!is_shadow_present_pte(iter.old_spte)) {

Would it be easier to reason about this code by making it retry, like:

retry:
                 if (is_shadow_present_pte(iter.old_spte)) {
			if (is_large_pte(iter.old_spte)) {
	                        if (!tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter))
	                                break;

				/*
				 * The iter must explicitly re-read the SPTE because
				 * the atomic cmpxchg failed.
				 */
	                        iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
				goto retry;
			}
                 } else {
			...
		}

?

Paolo