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

[Stefano Stabellini <sstabellini@kernel.org>]

On Wed, 22 Sep 2021, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> 
> 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 <oleksandr_andrushchenko@epam.com>

Please make this the first patch of the series so that...

> ---
> New in v2
> ---
>  drivers/xen/Kconfig               | 26 +++++++++++++++++++++++++-
>  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, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index 057ddf61ef61..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
> +
> +config XEN_PCIDEV_STUB
> +	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.
> +
>  config XEN_PCIDEV_BACKEND
>  	tristate "Xen PCI-device backend driver"
> -	depends on PCI && XEN
> +	depends on PCI && X86 && XEN
>  	depends on XEN_BACKEND

...we don't need this


> +	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 da34ce85dc88..bde63ef677b8 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;

Is this truly enough to stop the PV backend from initializing? Have you
actually tested it to make sure? If it works, amazing! I am quite happy
about this approach :-)




>  	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
>