linux-kernel - Re: [PATCH v3 06/37] KVM: x86: Consolidate logic for injecting page faults to L1

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Message-ID: <5225c99c-8231-ae7a-62d3-f461749da7d0@redhat.com>
Date:   Tue, 24 Mar 2020 01:47:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 06/37] KVM: x86: Consolidate logic for injecting page
 faults to L1

On 20/03/20 22:28, Sean Christopherson wrote:
> +void kvm_inject_l1_page_fault(struct kvm_vcpu *vcpu,
> +			      struct x86_exception *fault)
> +{
> +	vcpu->arch.mmu->inject_page_fault(vcpu, fault);
> +}
> +
>  bool kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
>  				    struct x86_exception *fault)
>  {
> @@ -619,7 +625,7 @@ bool kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
>  	if (mmu_is_nested(vcpu) && !fault->nested_page_fault)
>  		vcpu->arch.nested_mmu.inject_page_fault(vcpu, fault);
>  	else
> -		vcpu->arch.mmu->inject_page_fault(vcpu, fault);
> +		kvm_inject_l1_page_fault(vcpu, fault);
>  
>  	return fault->nested_page_fault;

This all started with "I don't like the name of the function" but
thinking more about it, we can also write this as

	if (mmu_is_nested(vcpu) && !fault->nested_page_fault)
		vcpu->arch.walk_mmu->inject_page_fault(vcpu, fault);
	else
		vcpu->arch.mmu->inject_page_fault(vcpu, fault);

Now, if !mmu_is_nested(vcpu) then walk_mmu == mmu, so it's much simpler
up until this patch:

	fault_mmu = fault->nested_page_fault ? vcpu->arch.mmu : vcpu->arch.walk_mmu;
	fault_mmu->inject_page_fault(vcpu, fault);

(which also matches how fault->nested_page_fault is assigned to).
In patch 7 we add the invalidation in kvm_inject_l1_page_fault, but
is it necessary to do it only in the else?

+	if (!vcpu->arch.mmu->direct_map &&
+	    (fault->error_code & PFERR_PRESENT_MASK))
+		vcpu->arch.mmu->invlpg(vcpu, fault->address,
+				       vcpu->arch.mmu->root_hpa);
+
 	vcpu->arch.mmu->inject_page_fault(vcpu, fault);
 }
 
The direct_map check is really just an optimization to avoid a
retpoline if ->invlpg is nonpaging_invlpg.  We can change it to
!vcpu->arch.mmu->invlpg if nonpaging_invlpg is replaced with NULL,
and then the same "if" condition can also be used for the nested_mmu
i.e. what patch 7 writes as

+		/*
+		 * No need to sync SPTEs, the fault is being injected into L2,
+		 * whose page tables are not being shadowed.
+		 */
 		vcpu->arch.nested_mmu.inject_page_fault(vcpu, fault);


Finally, patch 7 also adds a tlb_flush_gva call which is already present
in kvm_mmu_invlpg, and this brings the final form to look like this:

bool kvm_inject_emulated_page_fault(struct kvm_vcpu *vcpu,
                                    struct x86_exception *fault)
{
        struct kvm_mmu *fault_mmu;
        WARN_ON_ONCE(fault->vector != PF_VECTOR);

        fault_mmu = fault->nested_page_fault ? vcpu->arch.mmu : vcpu->arch.walk_mmu;

        /*
         * Invalidate the TLB entry for the faulting address, if it exists,
         * else the access will fault indefinitely (and to emulate hardware).
         */
        if (fault->error_code & PFERR_PRESENT_MASK)
                __kvm_mmu_invlpg(vcpu, fault_mmu, fault->address);

        fault_mmu->inject_page_fault(vcpu, fault);
        return fault->nested_page_fault;
}

This will become a formal mini-series replacing patches 6 and 7
after I test it, so no need to do anything on your part.

Paolo