Re: [PATCH] xen-pciback: allow compiling on other archs than x86

From: Juergen Gross
Date: Thu Nov 18 2021 - 03:14:49 EST

On 16.11.21 11:45, Oleksandr Andrushchenko wrote:
Hi, Geert!

On 16.11.21 11:36, Geert Uytterhoeven wrote:
Hi Oleksandr,

On Thu, Oct 28, 2021 at 8:15 AM Oleksandr Andrushchenko
<andr2000@xxxxxxxxx> wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Xen-pciback driver was designed to be built for x86 only. But it
can also be used by other architectures, e.g. Arm.

Currently PCI backend implements multiple functionalities at a time,
such as:
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

For that add new configuration options CONFIG_XEN_PCI_STUB and
CONFIG_XEN_PCIDEV_STUB, so the driver can be limited in its
functionality, e.g. no support for para-virtualised scenario.
x86 platform will continue using CONFIG_XEN_PCIDEV_BACKEND for the
fully featured backend driver.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@xxxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Thanks for your patch, which is now commit a67efff28832a597
("xen-pciback: allow compiling on other archs than x86")
in v5.16-rc1.

--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -181,10 +181,34 @@ 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
Please note that this means "default y" if CONFIG_MODULES=n.
Perhaps this should be "default m if MODULES" instead?
I don't really have strong opinion on this and will let Xen maintainers
speak: @Boris, @Juergen what's your preference here?

TBH, I don't have any.

All other backends have no "default" line, so maybe the cleanest
solution would be to remove the "default" lines for XEN_PCIDEV_STUB

+ 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


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