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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Date:   Fri, 18 Feb 2022 10:39:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH v2 15/18] KVM: x86/mmu: rename kvm_mmu_new_pgd, introduce
 variant that calls get_guest_pgd

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.

Paolo