Re: [PATCH v2 4/4] KVM: SVM: Support #GP handling for the case of nested on nested

[Maxim Levitsky]

On Thu, 2021-01-21 at 01:55 -0500, Wei Huang wrote:
> Under the case of nested on nested (e.g. 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.
> 
> Co-developed-by: Bandan Das <bsd@xxxxxxxxxx>
> Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
> Signed-off-by: Wei Huang <wei.huang2@xxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 2a12870ac71a..89512c0e7663 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2196,6 +2196,11 @@ 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,
> @@ -2203,7 +2208,14 @@ 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);
>  }
>  
>  /*
> @@ -4034,6 +4046,11 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  	/* Check again if INVPCID interception if required */
>  	svm_check_invpcid(svm);
>  
> +	if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM)) {
> +		best = kvm_find_cpuid_entry(vcpu, 0x8000000A, 0);
> +		best->edx |= (1 << 28);
> +	}
> +
>  	/* For sev guests, the memory encryption bit is not reserved in CR3.  */
>  	if (sev_guest(vcpu->kvm)) {
>  		best = kvm_find_cpuid_entry(vcpu, 0x8000001F, 0);

Tested-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>


Best regards,
	Maxim Levitsky