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 06:56:22 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Jan Beulich <jbeulich@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>
CC:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.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 28.09.21 09:42, Jan Beulich wrote:
> 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?

Exactly. I forgot that when playing with module/built-in I was not able

to control that anymore because CONFIG_XEN_PCI_STUB will always be

in "y" state, thus even if you have CONFIG_XEN_PCIDEV_BACKEND=m

you won't be able to build it as module. So, I will probably put a comment

about that in the Makefile explaining the need for

obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o

>
> Jan
>
Thank you,

Oleksandr