KVM: introduce new vendor op for KVM_GET_DEVICE_ATTR

Allow vendor modules to provide their own attributes on /dev/kvm.
To avoid proliferation of vendor ops, implement KVM_HAS_DEVICE_ATTR
and KVM_GET_DEVICE_ATTR in terms of the same function.  You're not
supposed to use KVM_GET_DEVICE_ATTR to do complicated computations,
especially on /dev/kvm.

Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-ID: <20240404121327.3107131-5-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm-x86-ops.h

@@ -121,6 +121,7 @@ KVM_X86_OP(enter_smm)
 KVM_X86_OP(leave_smm)
 KVM_X86_OP(enable_smi_window)
 #endif
+KVM_X86_OP_OPTIONAL(dev_get_attr)
 KVM_X86_OP_OPTIONAL(mem_enc_ioctl)
 KVM_X86_OP_OPTIONAL(mem_enc_register_region)
 KVM_X86_OP_OPTIONAL(mem_enc_unregister_region)

arch/x86/include/asm/kvm_host.h

@@ -1778,6 +1778,7 @@ struct kvm_x86_ops {
 	void (*enable_smi_window)(struct kvm_vcpu *vcpu);
 #endif
 
+	int (*dev_get_attr)(u32 group, u64 attr, u64 *val);
 	int (*mem_enc_ioctl)(struct kvm *kvm, void __user *argp);
 	int (*mem_enc_register_region)(struct kvm *kvm, struct kvm_enc_region *argp);
 	int (*mem_enc_unregister_region)(struct kvm *kvm, struct kvm_enc_region *argp);

arch/x86/kvm/x86.c

@@ -4842,34 +4842,44 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	return r;
 }
 
-static int kvm_x86_dev_get_attr(struct kvm_device_attr *attr)
+static int __kvm_x86_dev_get_attr(struct kvm_device_attr *attr, u64 *val)
 {
-	u64 __user *uaddr = u64_to_user_ptr(attr->addr);
-
-	if (attr->group)
+	if (attr->group) {
+		if (kvm_x86_ops.dev_get_attr)
+			return static_call(kvm_x86_dev_get_attr)(attr->group, attr->attr, val);
 		return -ENXIO;
+	}
 
 	switch (attr->attr) {
 	case KVM_X86_XCOMP_GUEST_SUPP:
-		if (put_user(kvm_caps.supported_xcr0, uaddr))
-			return -EFAULT;
+		*val = kvm_caps.supported_xcr0;
 		return 0;
 	default:
 		return -ENXIO;
 	}
 }
 
+static int kvm_x86_dev_get_attr(struct kvm_device_attr *attr)
+{
+	u64 __user *uaddr = u64_to_user_ptr(attr->addr);
+	int r;
+	u64 val;
+
+	r = __kvm_x86_dev_get_attr(attr, &val);
+	if (r < 0)
+		return r;
+
+	if (put_user(val, uaddr))
+		return -EFAULT;
+
+	return 0;
+}
+
 static int kvm_x86_dev_has_attr(struct kvm_device_attr *attr)
 {
-	if (attr->group)
-		return -ENXIO;
+	u64 val;
 
-	switch (attr->attr) {
-	case KVM_X86_XCOMP_GUEST_SUPP:
-		return 0;
-	default:
-		return -ENXIO;
-	}
+	return __kvm_x86_dev_get_attr(attr, &val);
 }
 
 long kvm_arch_dev_ioctl(struct file *filp,