linux-kernel - Re: [PATCH v2 10/18] KVM: x86/mmu: load new PGD after the shadow MMU is initialized

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Date:   Wed, 23 Feb 2022 18:20:39 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH v2 10/18] KVM: x86/mmu: load new PGD after the shadow
 MMU is initialized

On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
> Now that __kvm_mmu_new_pgd does not look at the MMU's root_level and
> shadow_root_level anymore, pull the PGD load after the initialization of
> the shadow MMUs.

Again, thanks a million for this! I once spend at least a hour figuring
out why my kernel panics when I did a similiar change, only to figure out
that __kvm_mmu_new_pgd needs to happen before mmu re-initialization.

> 
> Besides being more intuitive, this enables future simplifications
> and optimizations because it's not necessary anymore to compute the
> role outside kvm_init_mmu.  In particular, kvm_mmu_reset_context was not
> attempting to use a cached PGD to avoid having to figure out the new role.
> It will soon be able to follow what nested_{vmx,svm}_load_cr3 are doing,
> and avoid unloading all the cached roots.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/mmu/mmu.c    | 37 +++++++++++++++++--------------------
>  arch/x86/kvm/svm/nested.c |  6 +++---
>  arch/x86/kvm/vmx/nested.c |  6 +++---
>  3 files changed, 23 insertions(+), 26 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index da324a317000..906a9244ad28 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4903,9 +4903,8 @@ void kvm_init_shadow_npt_mmu(struct kvm_vcpu *vcpu, unsigned long cr0,
>  
>  	new_role = kvm_calc_shadow_npt_root_page_role(vcpu, &regs);
>  
> -	__kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base);
> -
>  	shadow_mmu_init_context(vcpu, context, &regs, new_role);
> +	__kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base);
>  }
>  EXPORT_SYMBOL_GPL(kvm_init_shadow_npt_mmu);
>  
> @@ -4943,27 +4942,25 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
>  		kvm_calc_shadow_ept_root_page_role(vcpu, accessed_dirty,
>  						   execonly, level);
>  
> -	__kvm_mmu_new_pgd(vcpu, new_eptp, new_role.base);
> -
> -	if (new_role.as_u64 == context->mmu_role.as_u64)
> -		return;
> -
> -	context->mmu_role.as_u64 = new_role.as_u64;
> +	if (new_role.as_u64 != context->mmu_role.as_u64) {
> +		context->mmu_role.as_u64 = new_role.as_u64;
>  
> -	context->shadow_root_level = level;
> +		context->shadow_root_level = level;
>  
> -	context->ept_ad = accessed_dirty;
> -	context->page_fault = ept_page_fault;
> -	context->gva_to_gpa = ept_gva_to_gpa;
> -	context->sync_page = ept_sync_page;
> -	context->invlpg = ept_invlpg;
> -	context->root_level = level;
> -	context->direct_map = false;
> +		context->ept_ad = accessed_dirty;
> +		context->page_fault = ept_page_fault;
> +		context->gva_to_gpa = ept_gva_to_gpa;
> +		context->sync_page = ept_sync_page;
> +		context->invlpg = ept_invlpg;
> +		context->root_level = level;
> +		context->direct_map = false;
> +		update_permission_bitmask(context, true);
> +		context->pkru_mask = 0;
> +		reset_rsvds_bits_mask_ept(vcpu, context, execonly, huge_page_level);
> +		reset_ept_shadow_zero_bits_mask(context, execonly);
> +	}
>  
> -	update_permission_bitmask(context, true);
> -	context->pkru_mask = 0;
> -	reset_rsvds_bits_mask_ept(vcpu, context, execonly, huge_page_level);
> -	reset_ept_shadow_zero_bits_mask(context, execonly);
> +	__kvm_mmu_new_pgd(vcpu, new_eptp, new_role.base);
>  }
>  EXPORT_SYMBOL_GPL(kvm_init_shadow_ept_mmu);
>  
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index f284e61451c8..96bab464967f 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -492,14 +492,14 @@ static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
>  	    CC(!load_pdptrs(vcpu, cr3)))
>  		return -EINVAL;
>  
> -	if (!nested_npt)
> -		kvm_mmu_new_pgd(vcpu, cr3);
> -
>  	vcpu->arch.cr3 = cr3;
>  
>  	/* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
>  	kvm_init_mmu(vcpu);
>  
> +	if (!nested_npt)
> +		kvm_mmu_new_pgd(vcpu, cr3);
> +
>  	return 0;
>  }
>  
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index b7bc634d35e2..1dfe23963a9e 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1126,15 +1126,15 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
>  		return -EINVAL;
>  	}
>  
> -	if (!nested_ept)
> -		kvm_mmu_new_pgd(vcpu, cr3);
> -
>  	vcpu->arch.cr3 = cr3;
>  	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
>  
>  	/* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
>  	kvm_init_mmu(vcpu);
>  
> +	if (!nested_ept)
> +		kvm_mmu_new_pgd(vcpu, cr3);
> +
>  	return 0;
>  }
>  


Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky