RE: [RFC v2 1/2] vfio/pci: export common symbols in vfio-pci

[Liu, Yi L]

> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Tuesday, March 26, 2019 11:35 PM
> To: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Subject: Re: [RFC v2 1/2] vfio/pci: export common symbols in vfio-pci
> 
> On Tue, 26 Mar 2019 12:37:37 +0000
> "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:
> 
> > > From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> > > Sent: Tuesday, March 26, 2019 2:17 AM
> > > To: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > > Subject: Re: [RFC v2 1/2] vfio/pci: export common symbols in vfio-pci
> > >
> > > On Sat, 23 Mar 2019 11:06:44 +0000
> > > "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote:
> > > > Hi Alex,
> >
> > [...]
> >
> > > >
> > > > I tried to get a common file which includes the definitions of the module
> > > > options and the common interfaces and get it linked separately with each
> > > > module. It works well when linked separately by config the
> > > > CONFIG_VFIO_PCI=m and CONFIG_VFIO_PCI_MDEV=m in kernel
> > > > configuration file. CONFIG_VFIO_PCI_MDEV is a new Kconfig macro
> > > > for the mdev wrapped version driver. However, if building the vfio-pci
> > > > and the mdev wrapped version into kernel image (config the
> > > > CONFIG_VFIO_PCI=y and CONFIG_VFIO_PCI_MDEV=y), then the symbols
> > > > defined in the common file will be shared thus doesn't allow dissimilar
> > > > user settings.
> > > >
> > > > Per my understanding, I think we expect to allow simultaneous usage of
> > > > the two drivers. So I think the way above doesn't meet our expectation.
> > >
> > > I agree.  They should be related in implementation only, from a user
> > > perspective they should be entirely separate.
> > >
> > > > I considered a possible proposal as below. May listen to your opinion
> > > > on it before heading to cook. Also, better idea is welcomed. :-)
> > > >
> > > > - get a common file includes interfaces which are common and have
> > > >   input parameters to differentiate the calling from vfio-pci and the
> > > >   wrapped version. e.g. vfio_pci_rw(). may call it as vfio_pci_common.c.
> > > >
> > > > - get another common file includes the definitions of the module options,
> > > >   and the functions which referred the options. Define all of them as static.
> > > >   may call it as common.c
> > > >
> > > > - get vfio_pci.c which includes the module_init/exit interfaces and driver
> > > >   registration operations of vfio-pci.ko. This file should include the
> common.c
> > > >   above to have same module options with the mdev wrapped version.
> > > >
> > > > - get vfio_pci_mdev.c which includes the module_init/exit interfaces and
> > > >   driver registration operations of vfio-pci-mdev.ko. It should also include
> > > >   the common.c above to have same module options with vfio-pci.ko.
> > > >
> > > > - Makefile:
> > > > vfio-pci-y := vfio_pci.o vfio_pci_common.o vfio_pci_intrs.o
> vfio_pci_rdwr.o
> > > vfio_pci_config.o
> > > > vfio-pci-$(CONFIG_VFIO_PCI_IGD) += vfio_pci_igd.o
> > > > vfio-pci-$(CONFIG_VFIO_PCI_NVLINK2) += vfio_pci_nvlink2.o
> > > >
> > > > vfio-pci-mdev-y := vfio_pci_mdev.o vfio_pci_common.o vfio_pci_intrs.o
> > > vfio_pci_rdwr.o vfio_pci_config.o
> > > > vfio-pci-mdev-$(CONFIG_VFIO_PCI_IGD) += vfio_pci_igd.o
> > > > vfio-pci-mdev-$(CONFIG_VFIO_PCI_NVLINK2) += vfio_pci_nvlink2.o
> > > >
> > > > obj-$(CONFIG_VFIO_PCI) += vfio-pci.o
> > > > obj-$(CONFIG_VFIO_PCI_MDEV) += vfio-pci-mdev.o
> > >
> > > Each module needs it's own module_init/exit and will register its own
> > > struct pci_driver, which gives us separate control of the probe and
> >
> > Agreed.
> >
> > > remove callbacks.  I think we want the drivers to have the same module
> > > parameters initially, but we don't necessarily want to require it for
> > > any future options, so we can duplicate the parameter declarations.
> > > Then to support the shared code, I think we can easily push nointxmask,
> > > disable_vga, and disable_idle_d3 into bools on the struct
> > > vfio_pci_device, which would be allocated and set by each module's
> > > probe function before calling the shared probe function.
> >
> > sounds good to me.
> >
> > > vfio_fill_ids() could take a pointer to the array to keep them separate
> > > between modules.
> >
> > Agreed.
> >
> > > I think that just leaves the config permission bits,
> > > vfio_pci_{un}init_perm_bits(). Could we use a simple atomic reference
> > > counter on those to potentially share them so they get initialized by
> > > the first caller and freed by the last user, at least in the case of
> > > both drivers being compiled statically into the kernel?  Thanks,
> >
> > Sure, I can add it. The two modules will still share the cap_perms and
> > ecap_perms config bits when built statically in kernel. However, I think
> > such share is reasonable. I'll check if any other similar bits in other files.
> >
> > > Alex
> >
> > Thanks for the suggestions, Alex. Let me prepare another RFC.
> 
> Thank Yi, I appreciate your work on this.  Also, I wonder if we might
> want to reconsider placing this driver in samples, the Makefile might
> be a little bit ugly with paths back to drivers/vfio/pci, but I don't
> think we run into the same barriers as you did with previous
> approaches.  Placing it in samples would at least alleviate any
> confusion that this isn't a vfio-pci replacement, but more of an mdev
> wrapper proof of concept.  Thanks,

Hi Alex,

My pleasure. Honestly, placing this driver in samples is also my first
choice at the beginning. I didn't go ahead with it due to the fear of
huge code duplication with drivers/vfio/pci. I can make the changes
based on the discussions in this thread and place the wrapped driver
under samples with its Makefile path back to drivers/vfio/pci. I believe
letting Makefile paths back to drivers/vfio/pci is fine as there is existing
case of such manner in kernel. Pls feel free to let me know your latest
idea or anything I missed. Thanks.

Regards,
Yi Liu