linux-kernel - Re: [PATCH 09/11] PCI: add matching checks for driver

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Date:   Tue, 15 Jun 2021 15:59:00 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Max Gurtovoy <mgurtovoy@...dia.com>, cohuck@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        aviadye@...dia.com, oren@...dia.com, shahafs@...dia.com,
        parav@...dia.com, artemp@...dia.com, kwankhede@...dia.com,
        ACurrid@...dia.com, cjia@...dia.com, yishaih@...dia.com,
        kevin.tian@...el.com, hch@...radead.org, targupta@...dia.com,
        shameerali.kolothum.thodi@...wei.com, liulongfang@...wei.com,
        yan.y.zhao@...el.com
Subject: Re: [PATCH 09/11] PCI: add matching checks for driver_override
 binding

On Tue, 15 Jun 2021 17:42:16 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Tue, Jun 15, 2021 at 10:20:49AM -0600, Alex Williamson wrote:
> > On Tue, 15 Jun 2021 12:04:58 -0300
> > Jason Gunthorpe <jgg@...dia.com> wrote:
> >   
> > > On Tue, Jun 15, 2021 at 09:00:29AM -0600, Alex Williamson wrote:
> > >   
> > > > "vfio" override in PCI-core plays out for other override types.  Also I
> > > > don't think dynamic IDs should be handled uniquely, new_id_store()
> > > > should gain support for flags and userspace should be able to add new
> > > > dynamic ID with override-only matches to the table.  Thanks,    
> > > 
> > > Why? Once all the enforcement is stripped out the only purpose of the
> > > new flag is to signal a different prepration of modules.alias - which
> > > won't happen for the new_id path anyhow  
> > 
> > Because new_id allows the admin to insert a new pci_device_id which has
> > been extended to include a flags field and intentionally handling
> > dynamic IDs differently from static IDs seems like generally a bad
> > thing.    
> 
> I'd agree with you if there was a functional difference at runtime,
> but since that was all removed, I don't think we should touch new_id.
> 
> This ends up effectively being only a kbuild related patch that
> changes how modules.alias is built.

But it wasn't all removed.  The proposal had:

 a) Short circuit the dynamic ID match
 b) Fail a driver-override-only match without a driver_override
 c) Fail a non-driver-override-only match with a driver_override

Max is only proposing removing c).

b) alone is a functional, runtime difference.  As per my previous
example, think about using it as effectively an anti-match.  Userspace
can create dynamic entries that will be matched before static entries
that can demote a driver to not be able to bind to a device
automatically.  That's functional and useful, I can't think of any
other way we have to effectively remove a static match entry from a
driver.  Thanks,

Alex