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

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Date:   Tue, 16 Nov 2021 10:36:15 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Oleksandr Andrushchenko <andr2000@...il.com>
Cc:     xen-devel@...ts.xenproject.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, julien@....org,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jan Beulich <jbeulich@...e.com>,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        Anastasiia Lukianenko <anastasiia_lukianenko@...m.com>
Subject: Re: [PATCH] xen-pciback: allow compiling on other archs than x86

Hi Oleksandr,

On Thu, Oct 28, 2021 at 8:15 AM Oleksandr Andrushchenko
<andr2000@...il.com> wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>
> 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@...m.com>
> Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@...m.com>
> Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>
> Reviewed-by: Juergen Gross <jgross@...e.com>

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?

> +       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

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds