linux-kernel - Re: [PATCH v2 15/18] KVM: x86/mmu: rename kvm_mmu_new_pgd, introduce variant that calls get_guest

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Message-ID: <YhAI2rq9ms+rhFy5@google.com>
Date:   Fri, 18 Feb 2022 21:00:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 15/18] KVM: x86/mmu: rename kvm_mmu_new_pgd, introduce
 variant that calls get_guest_pgd

On Fri, Feb 18, 2022, Paolo Bonzini wrote:
> On 2/17/22 22:03, Paolo Bonzini wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index adcee7c305ca..9800c8883a48 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1189,7 +1189,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
> >   		return 1;
> >   	if (cr3 != kvm_read_cr3(vcpu))
> > -		kvm_mmu_new_pgd(vcpu, cr3);
> > +		kvm_mmu_update_root(vcpu);
> >   	vcpu->arch.cr3 = cr3;
> >   	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> 
> Uh-oh, this has to become:
> 
>  	vcpu->arch.cr3 = cr3;
>  	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> 	if (!is_pae_paging(vcpu))
> 		kvm_mmu_update_root(vcpu);
> 
> The regression would go away after patch 16, but this is more tidy apart
> from having to check is_pae_paging *again*.
> 
> Incremental patch:
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index adcee7c305ca..0085e9fba372 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1188,11 +1189,11 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  	if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, cr3))
>  		return 1;
> -	if (cr3 != kvm_read_cr3(vcpu))
> -		kvm_mmu_update_root(vcpu);
> -
>  	vcpu->arch.cr3 = cr3;
>  	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> +	if (!is_pae_paging(vcpu))
> +		kvm_mmu_update_root(vcpu);
> +
>  	/* Do not call post_set_cr3, we do not get here for confidential guests.  */
> 
> An alternative is to move the vcpu->arch.cr3 update in load_pdptrs.
> Reviewers, let me know if you prefer that, then I'll send v3.

  c) None of the above.

MOV CR3 never requires a new root if TDP is enabled, and the guest_mmu is used if
and only if TDP is enabled.  Even when KVM intercepts CR3 when EPT=1 && URG=0, it
does so only to snapshot vcpu->arch.cr3, there's no need to get a new PGD.

Unless I'm missing something, your original suggestion of checking tdp_enabled is
the way to go.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6e0f7f22c6a7..2b02029c63d0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1187,7 +1187,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
        if (is_pae_paging(vcpu) && !load_pdptrs(vcpu, cr3))
                return 1;

-       if (cr3 != kvm_read_cr3(vcpu))
+       if (!tdp_enabled && cr3 != kvm_read_cr3(vcpu))
                kvm_mmu_new_pgd(vcpu, cr3);

        vcpu->arch.cr3 = cr3;