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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Message-ID: <31a86235-dd24-5215-907b-c05e356dee38@linux.alibaba.com>
Date:   Tue, 23 Nov 2021 17:34:34 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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>
Subject: Re: [PATCH 17/15] KVM: X86: Ensure pae_root to be reconstructed for
 shadow paging if the guest PDPTEs is changed


Hello, Paolo

any thought/concern about this one?

Thanks
Lai


On 2021/11/11 22:46, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> For shadow paging, the pae_root needs to be reconstructed before the
> coming VMENTER if the guest PDPTEs is changed.
> 
> But not all paths that call load_pdptrs() will cause the pae_root to be
> reconstructed. Normally, kvm_mmu_reset_context() and kvm_mmu_free_roots()
> are used to launch later reconstruction.
> 
> The commit d81135a57aa6("KVM: x86: do not reset mmu if CR0.CD and
> CR0.NW are changed") skips kvm_mmu_reset_context() after load_pdptrs()
> when changing CR0.CD and CR0.NW.
> 
> The commit 21823fbda552("KVM: x86: Invalidate all PGDs for the current
> PCID on MOV CR3 w/ flush") skips kvm_mmu_free_roots() after
> load_pdptrs() when rewriting the CR3 with the same value.
> 
> The commit a91a7c709600("KVM: X86: Don't reset mmu context when
> toggling X86_CR4_PGE") skips kvm_mmu_reset_context() after
> load_pdptrs() when changing CR4.PGE.
> 
> Normally, the guest doesn't change the PDPTEs before doing only the
> above operation without touching other bits that can force pae_root to
> be reconstructed.  Guests like linux would keep the PDPTEs unchaged
> for every instance of pagetable.
> 
> Fixes: d81135a57aa6("KVM: x86: do not reset mmu if CR0.CD and CR0.NW are changed")
> Fixes: 21823fbda552("KVM: x86: Invalidate all PGDs for the current PCID on MOV CR3 w/ flush")
> Fixes: a91a7c709600("KVM: X86: Don't reset mmu context when toggling X86_CR4_PGE")
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/x86.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0176eaa86a35..cfba337e46ab 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -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);
>   	}
>   	vcpu->arch.pdptrs_from_userspace = false;
>   
>