netdev - Re: [PATCH v14 vfio 6/8] vfio/pds: Add support for dirty page tracking

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Message-ID: <20230811095318.5ebcaa05.alex.williamson@redhat.com>
Date: Fri, 11 Aug 2023 09:53:18 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Christoph Hellwig <hch@....de>, "Tian, Kevin" <kevin.tian@...el.com>,
 Brett Creeley <bcreeley@....com>, Brett Creeley <brett.creeley@....com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "netdev@...r.kernel.org"
 <netdev@...r.kernel.org>, "yishaih@...dia.com" <yishaih@...dia.com>,
 "shameerali.kolothum.thodi@...wei.com"
 <shameerali.kolothum.thodi@...wei.com>, "horms@...nel.org"
 <horms@...nel.org>, "shannon.nelson@....com" <shannon.nelson@....com>
Subject: Re: [PATCH v14 vfio 6/8] vfio/pds: Add support for dirty page
 tracking

On Thu, 10 Aug 2023 15:11:43 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Thu, Aug 10, 2023 at 11:54:44AM -0600, Alex Williamson wrote:
> > On Thu, 10 Aug 2023 14:43:04 -0300
> > Jason Gunthorpe <jgg@...dia.com> wrote:
> >   
> > > On Thu, Aug 10, 2023 at 11:40:08AM -0600, Alex Williamson wrote:
> > >   
> > > > PCI Express® Base Specification Revision 6.0.1, pg 1461:
> > > > 
> > > >   9.3.3.11 VF Device ID (Offset 1Ah)
> > > > 
> > > >   This field contains the Device ID that should be presented for every VF to the SI.
> > > > 
> > > >   VF Device ID may be different from the PF Device ID...
> > > > 
> > > > That?  Thanks,    
> > > 
> > > NVMe matches using the class code, IIRC there is language requiring
> > > the class code to be the same.  
> > 
> > Ok, yes:
> > 
> >   7.5.1.1.6 Class Code Register (Offset 09h)
> >   ...
> >   The field in a PF and its associated VFs must return the same value
> >   when read.
> > 
> > Seems limiting, but it's indeed there.  We've got a lot of cleanup to
> > do if we're going to start rejecting drivers for devices with PCI
> > spec violations though ;)  Thanks,  
> 
> Well.. If we defacto say that Linux is endorsing ignoring this part of
> the spec then I predict we will see more vendors follow this approach.

The NVMe driver will claim PCI_CLASS_STORAGE_EXPRESS devices, but there
are also various vendor/device IDs in the table, some for the purpose
of setting driver data with quirks, some not.  So I think the spec
compliant behavior here would be that the VF replicates the PF class
code and we'd simply need to add the vendor/device explicitly to the id
table.

TBH, I can see why this spec requirement might get overlooked, it's a
rather arbitrary restriction of the VF device.  Thanks,

Alex