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

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Date:   Mon, 27 Sep 2021 07:35:33 +0000
From:   Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.com>
To:     Jan Beulich <jbeulich@...e.com>,
        Oleksandr Andrushchenko <andr2000@...il.com>
CC:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "julien@....org" <julien@....org>,
        "sstabellini@...nel.org" <sstabellini@...nel.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 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.

>
> Jan
>