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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

On 28.09.21 09:59, Juergen Gross wrote:
> On 28.09.21 08:56, Oleksandr Andrushchenko wrote:
>>
>> 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@epam.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@epam.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
>
> In case the real split between both parts of xen-pciback is done this
> will be needed anyway.

Yes, it will

So, I'll put a comment in the Makefile:

# N.B. This cannot be expressed with a single line using CONFIG_XEN_PCI_STUB

# as it always remains in "y" state, thus preventing the driver to be built as

# a module.

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

Will this be ok or needs some re-wording?

>
>
> Juergen
>
Thank you,

Oleksandr