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

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Message-ID: <750b9449-c986-025d-9b68-dd0f6967b083@suse.com>
Date:   Mon, 27 Sep 2021 09:48:53 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Oleksandr Andrushchenko <Oleksandr_Andrushchenko@...m.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 27.09.2021 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.

I'd suggest to do so, but I'm not the maintainer of this code.

Jan