Re: [PATCH 09/12] PCI: Add a PCI_ID_F_VFIO_DRIVER_OVERRIDE flag to struct pci_device_id

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Aug 13, 2021 at 02:21:41AM +0300, Max Gurtovoy wrote:
> 
> On 8/12/2021 11:26 PM, Bjorn Helgaas wrote:
> > On Thu, Aug 12, 2021 at 04:51:26PM -0300, Jason Gunthorpe wrote:
> > > On Thu, Aug 12, 2021 at 10:57:07AM -0500, Bjorn Helgaas wrote:
> > > > On Thu, Aug 12, 2021 at 10:27:28AM -0300, Jason Gunthorpe wrote:
> > > > > On Wed, Aug 11, 2021 at 02:07:37PM -0500, Bjorn Helgaas wrote:
> > > > > > On Thu, Aug 05, 2021 at 09:23:57PM -0300, Jason Gunthorpe wrote:
> > > > > > Do the other bus types have a flag analogous to
> > > > > > PCI_ID_F_VFIO_DRIVER_OVERRIDE?  If we're doing something similar to
> > > > > > other bus types, it'd be nice if the approach were similar.
> > > > > They could, this series doesn't attempt it. I expect the approach to
> > > > > be similar as driver_override was copied from PCI to other
> > > > > busses. When this is completed I hope to take a look at it.
> > > > I think this would make more sense as two patches:
> > > > 
> > > >    - Add a "PCI_ID_DRIVER_OVERRIDE" flag.  This is not VFIO-specific,
> > > >      since nothing in PCI depends on the VFIO-ness of drivers that use
> > > >      the flag.  The only point here is that driver id_table entries
> > > >      with this flag only match when driver_override matches the driver.
> > > This would require using two flags, one to indicate the above to the
> > > PCI code and another to indicate the vfio_pci string to
> > > file2alias. This doesn't seem justified at this point, IMHO.
> > I don't think it requires two flags.  do_pci_entry() has:
> > 
> >    if (flags & PCI_ID_F_VFIO_DRIVER_OVERRIDE)
> >      strcpy(alias, "vfio_pci:");
> > 
> > I'm just proposing a rename:
> > 
> > s/PCI_ID_F_VFIO_DRIVER_OVERRIDE/PCI_ID_DRIVER_OVERRIDE/
> > 
> > > >    - Update file2alias.c to export the flags and the "vfio_pci:" alias.
> > > >      This seems to be the only place where VFIO comes into play, and
> > > >      putting it in a separate patch will make it much smaller and it
> > > >      will be clear how it could be extended for other buses.
> > > Well, I don't want to see a flag called PCI_ID_DRIVER_OVERRIDE mapped
> > > to the string "vfio_pci", that is just really confusing.
> > Hahaha, I see, that's fair :)  It confused me for a long time why you
> > wanted "VFIO" in the flag name because from the kernel's point of
> > view, the flag is not related to any VFIO-ness.  It's only related to
> > a special variety of driver_override, and VFIO happens to be one user
> > of it.
> 
> In my original patch I used
> 
> #define PCI_ID_DRIVER_OVERRIDE PCI_ID_F_VFIO_DRIVER_OVERRIDE
> 
> and in the pci core code I used PCI_ID_DRIVER_OVERRIDE in the "if" clause.
> 
> So we can maybe do that and leave the option to future update of the define
> without changing the core code.
> 
> In the future we can have something like:
> 
> #define PCI_ID_DRIVER_OVERRIDE (PCI_ID_F_VFIO_DRIVER_OVERRIDE |
> PCI_ID_F_MY_BUS_DRIVER_OVERRIDE)
> 
> The file2alias.c still have to use the exact PCI_ID_F_VFIO_DRIVER_OVERRIDE
> flag to add "vfio_" prefix.
> 
> Is that better ?

I don't think it's worth having two separate #defines.  If we need
more in the future, we can add them when we need them.

What if we renamed "flags" to be specifically for this override case,
e.g., "override_only"?  Then the flag could be
PCI_ID_F_VFIO_DRIVER_OVERRIDE, which would trigger a "vfio_" prefix in
file2alias.c, but pci_match_device() could just check for it being
non-zero, without caring whether the reason is VFIO or something else,
e.g.,

  pci_match_device(...)
  {
    ...
    if (found_id->override_only) {
      if (dev->driver_override)
        return found_id;
      ...

Bjorn