linux-kernel - Re: [PATCH v4 5/9] KVM: nSVM: introduce nested_svm_load_cr3()/nested_npt

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Date:   Mon, 13 Jul 2020 15:38:14 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/9] KVM: nSVM: introduce
 nested_svm_load_cr3()/nested_npt_enabled()

On Fri, Jul 10, 2020 at 04:11:53PM +0200, Vitaly Kuznetsov wrote:
> As a preparatory change for implementing nested specifig PGD switch for

s/specifig/specific

> nSVM (following nVMX' nested_vmx_load_cr3()) instead of relying on

nVMX's

> kvm_set_cr3() introduce nested_svm_load_cr3().

The changelog isn't all that helpful to understanding the actual change.
All this is doing is wrapping kvm_set_cr3(), but that's not at all obvious
from reading the above.

E.g.

  Add nested_svm_load_cr3() as a pass-through to kvm_set_cr3() as a
  preparatory change for implementing nested specific PGD switch for nSVM,
  (following nVMx's nested_vmx_load_cr3()).

> No functional change intended.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 21 +++++++++++++++++++--
>  1 file changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 5e6c988a4e6b..180929f3dbef 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -311,6 +311,21 @@ static void nested_vmcb_save_pending_event(struct vcpu_svm *svm,
>  	nested_vmcb->control.exit_int_info = exit_int_info;
>  }
>  
> +static inline bool nested_npt_enabled(struct vcpu_svm *svm)
> +{
> +	return svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE;
> +}
> +
> +/*
> + * Load guest's cr3 at nested entry. @nested_npt is true if we are
> + * emulating VM-Entry into a guest with NPT enabled.
> + */
> +static int nested_svm_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
> +			       bool nested_npt)

IMO the addition of nested_npt_enabled() should be a separate patch, and
the additoin of @nested_npt should be in patch 7.

Hypothetically speaking, if nested_npt_enabled() is inaccurate at the call
site in nested_prepare_vmcb_save(), then this patch is technically wrong
even though it doesn't introduce a bug.  Given that the call site of
nested_svm_load_cr3() is moved in patch 7, I don't see any value in adding
the placeholder parameter early.

> +{
> +	return kvm_set_cr3(vcpu, cr3);
> +}
> +
>  static void nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_vmcb)
>  {
>  	/* Load the nested guest state */
> @@ -324,7 +339,8 @@ static void nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_v
>  	svm_set_efer(&svm->vcpu, nested_vmcb->save.efer);
>  	svm_set_cr0(&svm->vcpu, nested_vmcb->save.cr0);
>  	svm_set_cr4(&svm->vcpu, nested_vmcb->save.cr4);
> -	(void)kvm_set_cr3(&svm->vcpu, nested_vmcb->save.cr3);
> +	(void)nested_svm_load_cr3(&svm->vcpu, nested_vmcb->save.cr3,
> +				  nested_npt_enabled(svm));
>  
>  	svm->vmcb->save.cr2 = svm->vcpu.arch.cr2 = nested_vmcb->save.cr2;
>  	kvm_rax_write(&svm->vcpu, nested_vmcb->save.rax);
> @@ -343,7 +359,8 @@ static void nested_prepare_vmcb_save(struct vcpu_svm *svm, struct vmcb *nested_v
>  static void nested_prepare_vmcb_control(struct vcpu_svm *svm)
>  {
>  	const u32 mask = V_INTR_MASKING_MASK | V_GIF_ENABLE_MASK | V_GIF_MASK;
> -	if (svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE)
> +
> +	if (nested_npt_enabled(svm))
>  		nested_svm_init_mmu_context(&svm->vcpu);
>  
>  	/* Guest paging mode is active - reset mmu */
> -- 
> 2.25.4
>