linux-kernel - Re: [PATCH 25/30] KVM: nSVM: leave guest mode when clearing EFER.SVME

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Date:   Sun, 31 May 2020 19:26:35 -0700
From:   Krish Sadhukhan <krish.sadhukhan@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 25/30] KVM: nSVM: leave guest mode when clearing EFER.SVME


On 5/29/20 8:39 AM, Paolo Bonzini wrote:
> According to the AMD manual, the effect of turning off EFER.SVME while a
> guest is running is undefined.  We make it leave guest mode immediately,
> similar to the effect of clearing the VMX bit in MSR_IA32_FEAT_CTL.


I see that svm_set_efer() is called in enter_svm_guest_mode() and 
nested_svm_vmexit(). In the VMRUN path, we have already checked 
EFER.SVME in nested_vmcb_checks(). So if it was not set, we wouldn't 
come to enter_svm_guest_mode(). Your fix is only for the #VMEXIT path then ?

>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/svm/nested.c | 16 ++++++++++++++++
>   arch/x86/kvm/svm/svm.c    | 10 ++++++++--
>   arch/x86/kvm/svm/svm.h    |  1 +
>   3 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index bd3a89cd4070..369eca73fe3e 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -618,6 +618,22 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>   	return 0;
>   }
>   
> +/*
> + * Forcibly leave nested mode in order to be able to reset the VCPU later on.
> + */
> +void svm_leave_nested(struct vcpu_svm *svm)
> +{
> +	if (is_guest_mode(&svm->vcpu)) {
> +		struct vmcb *hsave = svm->nested.hsave;
> +		struct vmcb *vmcb = svm->vmcb;
> +
> +		svm->nested.nested_run_pending = 0;
> +		leave_guest_mode(&svm->vcpu);
> +		copy_vmcb_control_area(&vmcb->control, &hsave->control);
> +		nested_svm_uninit_mmu_context(&svm->vcpu);
> +	}
> +}
> +
>   static int nested_svm_exit_handled_msr(struct vcpu_svm *svm)
>   {
>   	u32 offset, msr, value;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index bc08221f6743..b4db9a980469 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -265,6 +265,7 @@ static int get_npt_level(struct kvm_vcpu *vcpu)
>   
>   void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>   {
> +	struct vcpu_svm *svm = to_svm(vcpu);
>   	vcpu->arch.efer = efer;
>   
>   	if (!npt_enabled) {
> @@ -275,8 +276,13 @@ void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>   			efer &= ~EFER_LME;
>   	}
>   
> -	to_svm(vcpu)->vmcb->save.efer = efer | EFER_SVME;
> -	mark_dirty(to_svm(vcpu)->vmcb, VMCB_CR);
> +	if (!(efer & EFER_SVME)) {
> +		svm_leave_nested(svm);
> +		svm_set_gif(svm, true);
> +	}
> +
> +	svm->vmcb->save.efer = efer | EFER_SVME;
> +	mark_dirty(svm->vmcb, VMCB_CR);
>   }
>   
>   static int is_external_interrupt(u32 info)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index be8e830f83fa..6ac4c00a5d82 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -389,6 +389,7 @@ static inline bool nested_exit_on_nmi(struct vcpu_svm *svm)
>   
>   void enter_svm_guest_mode(struct vcpu_svm *svm, u64 vmcb_gpa,
>   			  struct vmcb *nested_vmcb);
> +void svm_leave_nested(struct vcpu_svm *svm);
>   int nested_svm_vmrun(struct vcpu_svm *svm);
>   void nested_svm_vmloadsave(struct vmcb *from_vmcb, struct vmcb *to_vmcb);
>   int nested_svm_vmexit(struct vcpu_svm *svm);