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

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Date:   Thu, 18 Nov 2021 09:13:29 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.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>
Subject: Re: [PATCH] xen-pciback: allow compiling on other archs than x86

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@...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?

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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