linux-kernel - Re: [PATCH v2 11/17] KVM: x86/mmu: Mark the PAE roots as decrypted for shadow paging

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Message-ID: <d18e53f9-f8ce-9793-a3e7-ad945fd1f5f0@redhat.com>
Date:   Fri, 5 Mar 2021 18:44:18 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 11/17] KVM: x86/mmu: Mark the PAE roots as decrypted
 for shadow paging

On 05/03/21 02:10, Sean Christopherson wrote:
> @@ -5301,6 +5307,22 @@ static int __kvm_mmu_create(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
>   	for (i = 0; i < 4; ++i)
>   		mmu->pae_root[i] = 0;

I think this should be deleted, since you have another identical for 
loop below?

Paolo

> +	/*
> +	 * CR3 is only 32 bits when PAE paging is used, thus it's impossible to
> +	 * get the CPU to treat the PDPTEs as encrypted.  Decrypt the page so
> +	 * that KVM's writes and the CPU's reads get along.  Note, this is
> +	 * only necessary when using shadow paging, as 64-bit NPT can get at
> +	 * the C-bit even when shadowing 32-bit NPT, and SME isn't supported
> +	 * by 32-bit kernels (when KVM itself uses 32-bit NPT).
> +	 */
> +	if (!tdp_enabled)
> +		set_memory_decrypted((unsigned long)mmu->pae_root, 1);
> +	else
> +		WARN_ON_ONCE(shadow_me_mask);
> +
> +	for (i = 0; i < 4; ++i)
> +		mmu->pae_root[i] = 0;
> +