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

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Date:   Fri, 5 Mar 2021 10:02:36 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Fri, Mar 05, 2021, Paolo Bonzini wrote:
> 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?

Yes, rebase gone awry.  And the zeroing needs to be done after decryption.
Good eyes!

> 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;
> > +
>