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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 28 Sep 2021, 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@xxxxxxxx>
> >>>>>>>>
> >>>>>>>> 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@xxxxxxxx>
> >>>>>>>>
> >>>>>>>> ---
> >>>>>>>> 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 am fine with it and honestly that was the only comment I had so you
can add my

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>