linux-kernel - Re: [PATCH v3 4/4] KVM: SVM: Support #GP handling for the case of nested on nested

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Date:   Tue, 26 Jan 2021 13:59:48 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        vkuznets@...hat.com, seanjc@...gle.com, joro@...tes.org,
        bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, bsd@...hat.com,
        dgilbert@...hat.com, luto@...capital.net
Subject: Re: [PATCH v3 4/4] KVM: SVM: Support #GP handling for the case of
 nested on nested

On Tue, 2021-01-26 at 03:18 -0500, Wei Huang wrote:
> Under the case of nested on nested (L0->L1->L2->L3), #GP triggered by
> SVM instructions can be hided from L1. Instead the hypervisor can
> inject the proper #VMEXIT to inform L1 of what is happening. Thus L1
> can avoid invoking the #GP workaround. For this reason we turns on
> guest VM's X86_FEATURE_SVME_ADDR_CHK bit for KVM running inside VM to
> receive the notification and change behavior.
> 
> Similarly we check if vcpu is under guest mode before emulating the
> vmware-backdoor instructions. For the case of nested on nested, we
> let the guest handle it.
> 
> Co-developed-by: Bandan Das <bsd@...hat.com>
> Signed-off-by: Bandan Das <bsd@...hat.com>
> Signed-off-by: Wei Huang <wei.huang2@....com>
> Tested-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/svm/svm.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f9233c79265b..83c401d2709f 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -929,6 +929,9 @@ static __init void svm_set_cpu_caps(void)
>  
>  		if (npt_enabled)
>  			kvm_cpu_cap_set(X86_FEATURE_NPT);
> +
> +		/* Nested VM can receive #VMEXIT instead of triggering #GP */
> +		kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK);
>  	}
>  
>  	/* CPUID 0x80000008 */
> @@ -2198,6 +2201,11 @@ static int svm_instr_opcode(struct kvm_vcpu *vcpu)
>  
>  static int emulate_svm_instr(struct kvm_vcpu *vcpu, int opcode)
>  {
> +	const int guest_mode_exit_codes[] = {
> +		[SVM_INSTR_VMRUN] = SVM_EXIT_VMRUN,
> +		[SVM_INSTR_VMLOAD] = SVM_EXIT_VMLOAD,
> +		[SVM_INSTR_VMSAVE] = SVM_EXIT_VMSAVE,
> +	};
>  	int (*const svm_instr_handlers[])(struct vcpu_svm *svm) = {
>  		[SVM_INSTR_VMRUN] = vmrun_interception,
>  		[SVM_INSTR_VMLOAD] = vmload_interception,
> @@ -2205,7 +2213,14 @@ static int emulate_svm_instr(struct kvm_vcpu *vcpu, int opcode)
>  	};
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	return svm_instr_handlers[opcode](svm);
> +	if (is_guest_mode(vcpu)) {
> +		svm->vmcb->control.exit_code = guest_mode_exit_codes[opcode];
> +		svm->vmcb->control.exit_info_1 = 0;
> +		svm->vmcb->control.exit_info_2 = 0;
> +
> +		return nested_svm_vmexit(svm);
> +	} else
> +		return svm_instr_handlers[opcode](svm);
>  }
>  
>  /*
> @@ -2239,7 +2254,8 @@ static int gp_interception(struct vcpu_svm *svm)
>  		 * VMware backdoor emulation on #GP interception only handles
>  		 * IN{S}, OUT{S}, and RDPMC.
>  		 */
> -		return kvm_emulate_instruction(vcpu,
> +		if (!is_guest_mode(vcpu))
> +			return kvm_emulate_instruction(vcpu,
>  				EMULTYPE_VMWARE_GP | EMULTYPE_NO_DECODE);
>  	} else
>  		return emulate_svm_instr(vcpu, opcode);

To be honest I expected the vmware backdoor fix to be in a separate patch,
but I see that Paulo already took these patches so I guess it is too late.

Anyway I am very happy to see this workaround merged, and see that bug
disappear forever.

Best regards,
	Maxim Levitsky