Date:   Tue, 28 Sep 2021 09:20:14 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>,
        Jan Beulich <jbeulich@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Cc:     "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>
Subject: Re: [PATCH v4 1/2] xen-pciback: prepare for the split for stub and PV

On 28.09.21 09:17, Oleksandr Andrushchenko wrote:
> 
> 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@...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-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
>>
>> In case the real split between both parts of xen-pciback is done this
>> will be needed anyway.
> 
> Yes, it will
> 
> So, I'll put a comment in the Makefile:
> 
> # N.B. This cannot be expressed with a single line using CONFIG_XEN_PCI_STUB
> 
> # as it always remains in "y" state, thus preventing the driver to be built as
> 
> # a module.
> 
> obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
> obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
> 
> Will this be ok or needs some re-wording?

I'd add that CONFIG_XEN_PCIDEV_BACKEND and CONFIG_XEN_PCIDEV_STUB are
mutually exclusive.


Juergen

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