vfio-pci: Add VGA arbiter client

If VFIO VGA access is disabled for the user, either by CONFIG option
or module parameter, we can often opt-out of VGA arbitration.  We can
do this when PCI bridge control of VGA routing is possible.  This
means that we must have a parent bridge and there must only be a
single VGA device below that bridge.  Fortunately this is the typical
case for discrete GPUs.

Doing this allows us to minimize the impact of additional GPUs, in
terms of VGA arbitration, when they are only used via vfio-pci for
non-VGA applications.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci.c

@@ -25,6 +25,7 @@
 #include <linux/types.h>
 #include <linux/uaccess.h>
 #include <linux/vfio.h>
+#include <linux/vgaarb.h>
 
 #include "vfio_pci_private.h"
 
@@ -54,6 +55,50 @@ static inline bool vfio_vga_disabled(void)
 #endif
 }
 
+/*
+ * Our VGA arbiter participation is limited since we don't know anything
+ * about the device itself.  However, if the device is the only VGA device
+ * downstream of a bridge and VFIO VGA support is disabled, then we can
+ * safely return legacy VGA IO and memory as not decoded since the user
+ * has no way to get to it and routing can be disabled externally at the
+ * bridge.
+ */
+static unsigned int vfio_pci_set_vga_decode(void *opaque, bool single_vga)
+{
+	struct vfio_pci_device *vdev = opaque;
+	struct pci_dev *tmp = NULL, *pdev = vdev->pdev;
+	unsigned char max_busnr;
+	unsigned int decodes;
+
+	if (single_vga || !vfio_vga_disabled() || pci_is_root_bus(pdev->bus))
+		return VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM |
+		       VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM;
+
+	max_busnr = pci_bus_max_busnr(pdev->bus);
+	decodes = VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM;
+
+	while ((tmp = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, tmp)) != NULL) {
+		if (tmp == pdev ||
+		    pci_domain_nr(tmp->bus) != pci_domain_nr(pdev->bus) ||
+		    pci_is_root_bus(tmp->bus))
+			continue;
+
+		if (tmp->bus->number >= pdev->bus->number &&
+		    tmp->bus->number <= max_busnr) {
+			pci_dev_put(tmp);
+			decodes |= VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM;
+			break;
+		}
+	}
+
+	return decodes;
+}
+
+static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
+{
+	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
+}
+
 static void vfio_pci_try_bus_reset(struct vfio_pci_device *vdev);
 
 static int vfio_pci_enable(struct vfio_pci_device *vdev)
@@ -108,7 +153,7 @@ static int vfio_pci_enable(struct vfio_pci_device *vdev)
 	} else
 		vdev->msix_bar = 0xFF;
 
-	if (!vfio_vga_disabled() && (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA)
+	if (!vfio_vga_disabled() && vfio_pci_is_vga(pdev))
 		vdev->has_vga = true;
 
 	return 0;
@@ -900,6 +945,12 @@ static int vfio_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 		kfree(vdev);
 	}
 
+	if (vfio_pci_is_vga(pdev)) {
+		vga_client_register(pdev, vdev, NULL, vfio_pci_set_vga_decode);
+		vga_set_legacy_decoding(pdev,
+					vfio_pci_set_vga_decode(vdev, false));
+	}
+
 	return ret;
 }
 
@@ -908,9 +959,17 @@ static void vfio_pci_remove(struct pci_dev *pdev)
 	struct vfio_pci_device *vdev;
 
 	vdev = vfio_del_group_dev(&pdev->dev);
-	if (vdev) {
-		iommu_group_put(pdev->dev.iommu_group);
-		kfree(vdev);
+	if (!vdev)
+		return;
+
+	iommu_group_put(pdev->dev.iommu_group);
+	kfree(vdev);
+
+	if (vfio_pci_is_vga(pdev)) {
+		vga_client_register(pdev, NULL, NULL, NULL);
+		vga_set_legacy_decoding(pdev,
+				VGA_RSRC_NORMAL_IO | VGA_RSRC_NORMAL_MEM |
+				VGA_RSRC_LEGACY_IO | VGA_RSRC_LEGACY_MEM);
 	}
 }