[PATCH v4 1/2] xen-pciback: prepare for the split for stub and PV

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Currently PCI backend implements multiple functionalities at a time.
To name a few:
1. It is used as a database for assignable PCI devices, e.g. xl
   pci-assignable-{add|remove|list} manipulates that list. So, whenever
   the toolstack needs to know which PCI devices can be passed through
   it reads that from the relevant sysfs entries of the pciback.
2. It is used to hold the unbound PCI devices list, e.g. when passing
   through a PCI device it needs to be unbound from the relevant device
   driver and bound to pciback (strictly speaking it is not required
   that the device is bound to pciback, but pciback is again used as a
   database of the passed through PCI devices, so we can re-bind the
   devices back to their original drivers when guest domain shuts down)
3. Device reset for the devices being passed through
4. Para-virtualised use-cases support

The para-virtualised part of the driver is not always needed as some
architectures, e.g. Arm or x86 PVH Dom0, are not using backend-frontend
model for PCI device passthrough. For such use-cases make the very
first step in splitting the xen-pciback driver into two parts: Xen
PCI stub and PCI PV backend drivers.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

---
Changes since v3:
- Move CONFIG_XEN_PCIDEV_STUB to the second patch
Changes since v2:
 - swap the patch order
New in v2
---
 drivers/xen/Kconfig               | 4 ++++
 drivers/xen/Makefile              | 2 +-
 drivers/xen/xen-pciback/Makefile  | 1 +
 drivers/xen/xen-pciback/pciback.h | 5 +++++
 drivers/xen/xen-pciback/xenbus.c  | 6 +++++-
 5 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 22f5aff0c136..4efc95ddda41 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -181,10 +181,14 @@ config SWIOTLB_XEN
        select DMA_OPS
        select SWIOTLB
 
+config XEN_PCI_STUB
+       bool
+
 config XEN_PCIDEV_BACKEND
        tristate "Xen PCI-device backend driver"
        depends on PCI && X86 && XEN
        depends on XEN_BACKEND
+       select XEN_PCI_STUB
        default m
        help
          The PCI device backend driver allows the kernel to export arbitrary
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 3434593455b2..5aae66e638a7 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -24,7 +24,7 @@ obj-$(CONFIG_XEN_SYS_HYPERVISOR)      += sys-hypervisor.o
 obj-$(CONFIG_XEN_PVHVM_GUEST)          += platform-pci.o
 obj-$(CONFIG_SWIOTLB_XEN)              += swiotlb-xen.o
 obj-$(CONFIG_XEN_MCE_LOG)              += mcelog.o
-obj-$(CONFIG_XEN_PCIDEV_BACKEND)       += xen-pciback/
+obj-$(CONFIG_XEN_PCI_STUB)             += xen-pciback/
 obj-$(CONFIG_XEN_PRIVCMD)              += xen-privcmd.o
 obj-$(CONFIG_XEN_ACPI_PROCESSOR)       += xen-acpi-processor.o
 obj-$(CONFIG_XEN_EFI)                  += efi.o
diff --git a/drivers/xen/xen-pciback/Makefile b/drivers/xen/xen-pciback/Makefile
index e8d981d43235..e2cb376444a6 100644
--- a/drivers/xen/xen-pciback/Makefile
+++ b/drivers/xen/xen-pciback/Makefile
@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
+obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
 
 xen-pciback-y := pci_stub.o pciback_ops.o xenbus.o
 xen-pciback-y += conf_space.o conf_space_header.o \
diff --git a/drivers/xen/xen-pciback/pciback.h 
b/drivers/xen/xen-pciback/pciback.h
index 95e28ee48d52..9a64196e831d 100644
--- a/drivers/xen/xen-pciback/pciback.h
+++ b/drivers/xen/xen-pciback/pciback.h
@@ -71,6 +71,11 @@ struct pci_dev *pcistub_get_pci_dev(struct xen_pcibk_device 
*pdev,
                                    struct pci_dev *dev);
 void pcistub_put_pci_dev(struct pci_dev *dev);
 
+static inline bool xen_pcibk_pv_support(void)
+{
+       return IS_ENABLED(CONFIG_XEN_PCIDEV_BACKEND);
+}
+
 /* Ensure a device is turned off or reset */
 void xen_pcibk_reset_device(struct pci_dev *pdev);
 
diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c
index c09c7ebd6968..f8ba2903a3ff 100644
--- a/drivers/xen/xen-pciback/xenbus.c
+++ b/drivers/xen/xen-pciback/xenbus.c
@@ -743,6 +743,9 @@ const struct xen_pcibk_backend *__read_mostly 
xen_pcibk_backend;
 
 int __init xen_pcibk_xenbus_register(void)
 {
+       if (!xen_pcibk_pv_support())
+               return 0;
+
        xen_pcibk_backend = &xen_pcibk_vpci_backend;
        if (passthrough)
                xen_pcibk_backend = &xen_pcibk_passthrough_backend;
@@ -752,5 +755,6 @@ int __init xen_pcibk_xenbus_register(void)
 
 void __exit xen_pcibk_xenbus_unregister(void)
 {
-       xenbus_unregister_driver(&xen_pcibk_driver);
+       if (xen_pcibk_pv_support())
+               xenbus_unregister_driver(&xen_pcibk_driver);
 }
-- 
2.25.1