linux-kernel - Re: [PATCH 17/15] KVM: X86: Ensure pae_root to be reconstructed for shadow paging if the guest PDPTEs is changed

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Date:   Thu, 9 Dec 2021 23:46:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Subject: Re: [PATCH 17/15] KVM: X86: Ensure pae_root to be reconstructed for
 shadow paging if the guest PDPTEs is changed

On 12/8/21 01:15, Sean Christopherson wrote:
>> @@ -832,8 +832,14 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
>>   	if (memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs))) {
>>   		memcpy(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs));
>>   		kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
>> -		/* Ensure the dirty PDPTEs to be loaded. */
>> -		kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
>> +		/*
>> +		 * Ensure the dirty PDPTEs to be loaded for VMX with EPT
>> +		 * enabled or pae_root to be reconstructed for shadow paging.
>> +		 */
>> +		if (tdp_enabled)
>> +			kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
>> +		else
>> +			kvm_mmu_free_roots(vcpu, vcpu->arch.mmu, KVM_MMU_ROOT_CURRENT);
> Shouldn't matter since it's legacy shadow paging, but @mmu should be used instead
> of vcpu->arch.mmuvcpu->arch.mmu.

In kvm/next actually there's no mmu parameter to load_pdptrs, so it's 
okay to keep vcpu->arch.mmu.

> To avoid a dependency on the previous patch, I think it makes sense to have this be:
> 
> 	if (!tdp_enabled && memcmp(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs)))
> 		kvm_mmu_free_roots(vcpu, mmu, KVM_MMU_ROOT_CURRENT);
> 
> before the memcpy().
> 
> Then we can decide independently if skipping the KVM_REQ_LOAD_MMU_PGD if the
> PDPTRs are unchanged with respect to the MMU is safe.

Do you disagree that there's already an invariant that the PDPTRs can 
only be dirty if KVM_REQ_LOAD_MMU_PGD---and therefore a previous change 
to the PDPTRs would have triggered KVM_REQ_LOAD_MMU_PGD?  This is 
opposed to the guest TLB flush due to MOV CR3; that one has to be done 
unconditionally for PAE paging, and it is handled separately within 
kvm_set_cr3.

Paolo