vfio/pci: introduce CONFIG_VFIO_PCI_ZDEV_KVM

The current contents of vfio-pci-zdev are today only useful in a KVM
environment; let's tie everything currently under vfio-pci-zdev to
this Kconfig statement and require KVM in this case, reducing complexity
(e.g. symbol lookups).

Signed-off-by: Matthew Rosato <[e-mail]>
Acked-by: Alex Williamson <[e-mail]>
Reviewed-by: Pierre Morel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Borntraeger <[e-mail]>

drivers/vfio/pci/Kconfig

···
 	  To enable Intel IGD assignment through vfio-pci, say Y.
 endif
 
+config VFIO_PCI_ZDEV_KVM
+	bool "VFIO PCI extensions for s390x KVM passthrough"
+	depends on S390 && KVM
+	default y
+	help
+	  Support s390x-specific extensions to enable support for enhancements
+	  to KVM passthrough capabilities, such as interpretive execution of
+	  zPCI instructions.
+
+	  To enable s390x KVM vfio-pci extensions, say Y.
+
 source "drivers/vfio/pci/mlx5/Kconfig"
 
 source "drivers/vfio/pci/hisilicon/Kconfig"

drivers/vfio/pci/Makefile

···
 # SPDX-License-Identifier: GPL-2.0-only
 
 vfio-pci-core-y := vfio_pci_core.o vfio_pci_intrs.o vfio_pci_rdwr.o vfio_pci_config.o
-vfio-pci-core-$(CONFIG_S390) += vfio_pci_zdev.o
+vfio-pci-core-$(CONFIG_VFIO_PCI_ZDEV_KVM) += vfio_pci_zdev.o
 obj-$(CONFIG_VFIO_PCI_CORE) += vfio-pci-core.o
 
 vfio-pci-y := vfio_pci.o

include/linux/vfio_pci_core.h

···
 }
 #endif
 
-#ifdef CONFIG_S390
+#ifdef CONFIG_VFIO_PCI_ZDEV_KVM
 extern int vfio_pci_info_zdev_add_caps(struct vfio_pci_core_device *vdev,
 				       struct vfio_info_cap *caps);
 #else