vfio, platform: make reset driver a requirement by default

The code was allowing platform devices to be used without a supporting
VFIO reset driver. The hardware can be left in some inconsistent state
after a guest machine abort.

The reset driver will put the hardware back to safe state and disable
interrupts before returning the control back to the host machine.

Adding a new reset_required kernel module option to platform VFIO drivers.
The default value is true for the DT and ACPI based drivers.
The reset requirement value for AMBA drivers is set to false and is
unchangeable to maintain the existing functionality.

New requirements are:
1. A reset function needs to be implemented by the corresponding driver
via DT/ACPI.
2. The reset function needs to be discovered via DT/ACPI.

The probe of the driver will fail if any of the above conditions are
not satisfied.

Signed-off-by: Sinan Kaya <[e-mail]>
Signed-off-by: Alex Williamson <[e-mail]>

drivers/vfio/platform/vfio_amba.c

···
 	vdev->get_resource = get_amba_resource;
 	vdev->get_irq = get_amba_irq;
 	vdev->parent_module = THIS_MODULE;
+	vdev->reset_required = false;
 
 	ret = vfio_platform_probe_common(vdev, &adev->dev);
 	if (ret) {

drivers/vfio/platform/vfio_platform.c

···
 #define DRIVER_AUTHOR   "Antonios Motakis <[e-mail]>"
 #define DRIVER_DESC     "VFIO for platform devices - User Level meta-driver"
 
+static bool reset_required = true;
+module_param(reset_required, bool, 0444);
+MODULE_PARM_DESC(reset_required, "override reset requirement (default: 1)");
+
 /* probing devices from the linux platform bus */
 
 static struct resource *get_platform_resource(struct vfio_platform_device *vdev,
···
 	vdev->get_resource = get_platform_resource;
 	vdev->get_irq = get_platform_irq;
 	vdev->parent_module = THIS_MODULE;
+	vdev->reset_required = reset_required;
 
 	ret = vfio_platform_probe_common(vdev, &pdev->dev);
 	if (ret)

drivers/vfio/platform/vfio_platform_common.c

···
 	return vdev->of_reset ? true : false;
 }
 
-static void vfio_platform_get_reset(struct vfio_platform_device *vdev)
+static int vfio_platform_get_reset(struct vfio_platform_device *vdev)
 {
 	if (VFIO_PLATFORM_IS_ACPI(vdev))
-		return;
+		return vfio_platform_acpi_has_reset(vdev) ? 0 : -ENOENT;
 
 	vdev->of_reset = vfio_platform_lookup_reset(vdev->compat,
 						    &vdev->reset_module);
···
 		vdev->of_reset = vfio_platform_lookup_reset(vdev->compat,
 							&vdev->reset_module);
 	}
+
+	return vdev->of_reset ? 0 : -ENOENT;
 }
 
 static void vfio_platform_put_reset(struct vfio_platform_device *vdev)
···
 
 	vdev->device = dev;
 
+	ret = vfio_platform_get_reset(vdev);
+	if (ret && vdev->reset_required) {
+		pr_err("vfio: no reset function found for device %s\n",
+		       vdev->name);
+		return ret;
+	}
+
 	group = vfio_iommu_group_get(dev);
 	if (!group) {
 		pr_err("VFIO: No IOMMU group for device %s\n", vdev->name);
···
 		vfio_iommu_group_put(group, dev);
 		return ret;
 	}
-
-	vfio_platform_get_reset(vdev);
 
 	mutex_init(&vdev->igate);
 

drivers/vfio/platform/vfio_platform_private.h

···
 		(*get_resource)(struct vfio_platform_device *vdev, int i);
 	int	(*get_irq)(struct vfio_platform_device *vdev, int i);
 	int	(*of_reset)(struct vfio_platform_device *vdev);
+
+	bool				reset_required;
 };
 
 typedef int (*vfio_platform_reset_fn_t)(struct vfio_platform_device *vdev);