Re: [PATCH v4 2/9] KVM: SVM: Refactor SVM instruction handling on #GP intercept

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 16, 2026, Yosry Ahmed wrote:
> +/* Return 0 if not SVM instr, otherwise return associated exit_code */
> +static u64 svm_instr_exit_code(struct kvm_vcpu *vcpu)

To make it very clear what this is doing how about:

static u64 svm_get_decoded_instr_exit_code(struct kvm_vcpu *vcpu)

>  {
>  	struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
>  
>  	if (ctxt->b != 0x1 || ctxt->opcode_len != 2)
> -		return NONE_SVM_INSTR;
> +		return 0;

This should assert to ensure there's no collision with '0', i.e.

	BUILD_BUG_ON(!SVM_EXIT_VMRUN || !SVM_EXIT_VMLOAD || !SVM_EXIT_VMSAVE);


> +	if (!is_guest_mode(vcpu))
> +		return kvm_emulate_instruction(vcpu,
> +				EMULTYPE_VMWARE_GP | EMULTYPE_NO_DECODE);

Since you're moving this anyways:

	if (!is_guest_mode(vcpu))
		return kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE_GP |
						     EMULTYPE_NO_DECODE);


Actually!  Better idea, for this code and for the page_address_valid() checks:
invert the checks to reduce indentation, i.e. end up with:

	/* FIXME: Handle SVM instructions through the emulator */
	svm_exit_code = svm_get_decoded_instr_exit_code(vcpu);
	if (svm_exit_code) {
		if (!is_guest_mode(vcpu))
			return svm_invoke_exit_handler(vcpu, svm_exit_code);

		if (nested_svm_check_permissions(vcpu))
			return 1;

		if (!page_address_valid(vcpu, kvm_register_read(vcpu, VCPU_REGS_RAX)))
			goto reinject;

		/*
		 * FIXME: Only synthesize a #VMEXIT if L1 sets the intercept,
		 * but only after the VMLOAD/VMSAVE exit handlers can properly
		 * handle VMLOAD/VMSAVE from L2 with VLS enabled in L1 (i.e.
		 * RAX is an L2 GPA that needs translation through L1's NPT).
		 */
		nested_svm_simple_vmexit(svm, svm_exit_code);
		return 1;
	}

	/*
	 * VMware backdoor emulation on #GP interception only handles
	 * IN{S}, OUT{S}, and RDPMC, and only for L1.
	 */
	if (!enable_vmware_backdoor || is_guest_mode(vcpu))
		goto reinject;

	return kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE_GP | EMULTYPE_NO_DECODE);

> +
>  reinject:
>  	kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
>  	return 1;
> -- 
> 2.53.0.851.ga537e3e6e9-goog
>