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

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Message-ID: <a30fc7d9-0827-4970-845f-00cd711ff747@suse.com>
Date:   Tue, 28 Sep 2021 09:26:06 +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 09:24, Oleksandr Andrushchenko wrote:
> 
> On 28.09.21 10:20, Juergen Gross wrote:
>> 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-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o
>>>
>>> Will this be ok or needs some re-wording?
>>
>> I'd add that CONFIG_XEN_PCIDEV_BACKEND and CONFIG_XEN_PCIDEV_STUB are
>> mutually exclusive.
> # N.B. The below cannot be expressed with a single line using
> # CONFIG_XEN_PCI_STUB as it always remains in "y" state,
> # thus preventing the driver to be built as a module.
> # Please note, that CONFIG_XEN_PCIDEV_BACKEND and
> # CONFIG_XEN_PCIDEV_STUB are mutually exclusive.
> obj-$(CONFIG_XEN_PCIDEV_BACKEND) += xen-pciback.o
> obj-$(CONFIG_XEN_PCIDEV_STUB) += xen-pciback.o

Yes, that's fine.


Juergen


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