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

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Date: Fri, 11 Aug 2023 03:25:21 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>, Alex Williamson
	<alex.williamson@...hat.com>
CC: Christoph Hellwig <hch@....de>, 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

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, August 11, 2023 2:12 AM
> 
> 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.
> 

Looks PCI core assumes the class code must be same across VFs (though
not cross PF/VF). And it even violates the spec to require Revision ID
and Subsystem ID must be same too:

static void pci_read_vf_config_common(struct pci_dev *virtfn)
{
        struct pci_dev *physfn = virtfn->physfn;

        /*
         * Some config registers are the same across all associated VFs.
         * Read them once from VF0 so we can skip reading them from the
         * other VFs.
         *
         * PCIe r4.0, sec 9.3.4.1, technically doesn't require all VFs to
         * have the same Revision ID and Subsystem ID, but we assume they
         * do.
         */
        pci_read_config_dword(virtfn, PCI_CLASS_REVISION,
                              &physfn->sriov->class);
        pci_read_config_byte(virtfn, PCI_HEADER_TYPE,
                             &physfn->sriov->hdr_type);
        pci_read_config_word(virtfn, PCI_SUBSYSTEM_VENDOR_ID,
                             &physfn->sriov->subsystem_vendor);
        pci_read_config_word(virtfn, PCI_SUBSYSTEM_ID,
                             &physfn->sriov->subsystem_device);
}

Does AMD distributed card provide multiple PF's each for a class of
VF's or a single PF for all VF's?