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

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Message-ID: <ab6a44b3-6b56-191a-a653-ce5ace50975d@epam.com>
Date:   Tue, 16 Nov 2021 10:45:43 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
CC:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "julien@....org" <julien@....org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Jan Beulich <jbeulich@...e.com>,
        Anastasiia Lukianenko <Anastasiia_Lukianenko@...m.com>,
        Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
Subject: Re: [PATCH] xen-pciback: allow compiling on other archs than x86

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@...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?
I don't really have strong opinion on this and will let Xen maintainers
speak: @Boris, @Juergen what's your preference here?
>
>> +       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
Thank you,
Oleksandr
>
> --
> 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