linux-kernel - Re: [PATCH 5/5] KVM: x86/mmu: Use dummy root, backed by zero page, for !visible guest roots

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Date:   Tue, 25 Jul 2023 19:36:29 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Reima Ishii <ishiir@...cc.u-tokyo.ac.jp>
Subject: Re: [PATCH 5/5] KVM: x86/mmu: Use dummy root, backed by zero page,
 for !visible guest roots

> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 122bfc0124d3..e9d4d7b66111 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -646,6 +646,17 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
>  	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
>  		goto out_gpte_changed;
>  
> +	/*
> +	 * Load a new root and retry the faulting instruction in the extremely
> +	 * unlikely scenario that the guest root gfn became visible between
> +	 * loading a dummy root and handling the resulting page fault, e.g. if
> +	 * userspace create a memslot in the interim.
> +	 */
> +	if (unlikely(kvm_mmu_is_dummy_root(vcpu->arch.mmu->root.hpa))) {
> +		kvm_mmu_unload(vcpu);

Do we really need a kvm_mmu_unload()? Could we just set
vcpu->arch.mmu->root.hpa to INVALID_PAGE here? 

> +		goto out_gpte_changed;
> +	}
> +
>  	for_each_shadow_entry(vcpu, fault->addr, it) {
>  		gfn_t table_gfn;

B.R.
Yu