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Date:   Thu, 23 Sep 2021 12:53:44 +0300
From:   Oleksandr Andrushchenko <>
        Oleksandr Andrushchenko <>
Subject: [PATCH v3 1/2] xen-pciback: prepare for the split for stub and PV

From: Oleksandr Andrushchenko <>

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-virtualized use-cases support

The para-virtualized 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: extended
PCI stub and PCI PV backend drivers. At the moment x86 platform will
continue using CONFIG_XEN_PCIDEV_BACKEND for the fully featured backend
driver and new platforms may build a driver with limited functionality
(no PV) by enabling CONFIG_XEN_PCIDEV_STUB.

Signed-off-by: Oleksandr Andrushchenko <>

Changes since v2:
 - swap the patch order
New in v2
 drivers/xen/Kconfig               | 24 ++++++++++++++++++++++++
 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, 36 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index a37eb52fb401..6e92c6be19f1 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -180,10 +180,34 @@ config SWIOTLB_XEN
 	select DMA_OPS
 	select SWIOTLB
+config XEN_PCI_STUB
+	bool
+	tristate "Xen PCI-device stub driver"
+	depends on PCI && !X86 && XEN
+	depends on XEN_BACKEND
+	select XEN_PCI_STUB
+	default m
+	help
+	  The PCI device stub driver provides limited version of the PCI
+	  device backend driver without para-virtualized support for guests.
+	  If you select this to be a module, you will need to make sure no
+	  other driver has bound to the device(s) you want to make visible to
+	  other guests.
+	  The "hide" parameter (only applicable if backend driver is compiled
+	  into the kernel) allows you to bind the PCI devices to this module
+	  from the default device drivers. The argument is the list of PCI BDFs:
+	  xen-pciback.hide=(03:00.0)(04:00.0)
+	  If in doubt, say m.
 	tristate "Xen PCI-device backend driver"
 	depends on PCI && X86 && XEN
 	depends on XEN_BACKEND
+	select XEN_PCI_STUB
 	default m
 	  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)
 /* 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);

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