linux-kernel - Re: [PATCH v4 1/2] xen-pciback: prepare for the split for stub and PV

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Date:   Tue, 28 Sep 2021 08:42:07 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>,
        Oleksandr Andrushchenko <andr2000@...il.com>,
        "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "julien@....org" <julien@....org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH v4 1/2] xen-pciback: prepare for the split for stub and PV

On 28.09.2021 06:18, Stefano Stabellini wrote:
> On Mon, 27 Sep 2021, Juergen Gross wrote:
>> On 27.09.21 09:35, Oleksandr Andrushchenko wrote:
>>>
>>> On 27.09.21 10:26, Jan Beulich wrote:
>>>> On 27.09.2021 08:58, Oleksandr Andrushchenko wrote:
>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
>>>>>
>>>>> Currently PCI backend implements multiple functionalities at a time.
>>>>> To name a few:
>>>>> 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.
>>>>>
>>>>> Signed-off-by: Oleksandr Andrushchenko
>>>>> <oleksandr_andrushchenko@...m.com>
>>>>>
>>>>> ---
>>>>> Changes since v3:
>>>>> - Move CONFIG_XEN_PCIDEV_STUB to the second patch
>>>> I'm afraid this wasn't fully done:
>>>>
>>>>> --- a/drivers/xen/xen-pciback/Makefile
>>>>> +++ b/drivers/xen/xen-pciback/Makefile
>>>>> @@ -1,5 +1,6 @@
>>>>>    # SPDX-License-Identifier: GPL-2.0
>>>>>    obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
>>>>> +obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
>>>> While benign when added here, this addition still doesn't seem to
>>>> belong here.
>>>
>>> My bad. So, it seems without CONFIG_XEN_PCIDEV_STUB the change seems
>>>
>>> to be non-functional. With CONFIG_XEN_PCIDEV_STUB we fail to build on 32-bit
>>>
>>> architectures...
>>>
>>> What would be the preference here? Stefano suggested that we still define
>>>
>>> CONFIG_XEN_PCIDEV_STUB, but in disabled state, e.g. we add tristate to it
>>>
>>> in the second patch
>>>
>>> Another option is just to squash the two patches.
>>
>> Squashing would be fine for me.
>  
> It is fine for me to squash the two patches.
> 
> But in any case, wouldn't it be better to modify that specific change to:
> 
> diff --git a/drivers/xen/xen-pciback/Makefile b/drivers/xen/xen-pciback/Makefile
> index e2cb376444a6..e23c758b85ae 100644
> --- a/drivers/xen/xen-pciback/Makefile
> +++ b/drivers/xen/xen-pciback/Makefile
> @@ -1,6 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
> -obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
> -obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
> +obj-$(CONFIG_XEN_PCI_STUB) += xen-pciback.o

But that wouldn't allow the driver to be a module anymore, would it?

Jan