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 08:59:07 +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 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.


Juergen


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