linux-kernel - Re: [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional paging is initialized

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Message-ID: <Yf178LYEY4pFJcLc@google.com>
Date:   Fri, 4 Feb 2022 19:18:08 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, vkuznets@...hat.com
Subject: Re: [PATCH 06/23] KVM: MMU: load new PGD once nested two-dimensional
 paging is initialized

On Fri, Feb 04, 2022 at 06:57:01AM -0500, Paolo Bonzini wrote:
> __kvm_mmu_new_pgd looks at the MMU's root_level and shadow_root_level
> via fast_pgd_switch.

Those checks are just for performance correct (to skip iterating through
the list of roots)?

Either way, it's probably worth including a Fixes tag below.

> It makes no sense to call it before updating
> these fields, even though it was done like that ever since nested
> VMX grew the ability to use fast CR3 switch (commit 50c28f21d045,
> "kvm: x86: Use fast CR3 switch for nested VMX").
> 
> Pull it to the end of the initialization of the shadow nested MMUs.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Reviewed-by: David Matlack <dmatlack@...gle.com>

> ---
>  arch/x86/kvm/mmu/mmu.c | 41 +++++++++++++++++++----------------------
>  1 file changed, 19 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 577e70509510..b8ab16323629 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4869,10 +4869,9 @@ 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);
>  	reset_shadow_zero_bits_mask(vcpu, context, is_efer_nx(context));
> +	__kvm_mmu_new_pgd(vcpu, nested_cr3, new_role.base);
>  }
>  EXPORT_SYMBOL_GPL(kvm_init_shadow_npt_mmu);
>  
> @@ -4906,27 +4905,25 @@ 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;
> -
> -	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;
> +	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->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);
>  
> -- 
> 2.31.1
> 
>