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

[Juergen Gross]

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
> > > drivers.
> > >
> > > 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
> > > +
> > > +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
> > 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
and XEN_PCIDEV_BACKEND, too?

> > > +       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 && X86 && XEN
> > >           depends on XEN_BACKEND
> > > +       select XEN_PCI_STUB
> > >           default m
> > >           help
> > >             The PCI device backend driver allows the kernel to export arbitrary


Juergen

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