Re: [PATCH 09/11] PCI: add matching checks for driver_override binding

From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Max Gurtovoy <mgurtovoy@nvidia.com>,
	cohuck@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, aviadye@nvidia.com,
	oren@nvidia.com, shahafs@nvidia.com, parav@nvidia.com,
	artemp@nvidia.com, kwankhede@nvidia.com, ACurrid@nvidia.com,
	cjia@nvidia.com, yishaih@nvidia.com, kevin.tian@intel.com,
	hch@infradead.org, targupta@nvidia.com,
	shameerali.kolothum.thodi@huawei.com, liulongfang@huawei.com,
	yan.y.zhao@intel.com
Subject: Re: [PATCH 09/11] PCI: add matching checks for driver_override binding
Date: Tue, 15 Jun 2021 15:59:00 -0600	[thread overview]
Message-ID: <20210615155900.51f09c15.alex.williamson@redhat.com> (raw)
In-Reply-To: <20210615204216.GY1002214@nvidia.com>

On Tue, 15 Jun 2021 17:42:16 -0300
Jason Gunthorpe <jgg@nvidia.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@nvidia.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