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

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Message-ID: <BN9PR11MB527661C57FAFB0C30931339F8C14A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Tue, 15 Aug 2023 02:45:59 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Brett Creeley <bcreeley@....com>, Jason Gunthorpe <jgg@...dia.com>, "Alex
 Williamson" <alex.williamson@...hat.com>
CC: Christoph Hellwig <hch@....de>, 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: Brett Creeley <bcreeley@....com>
> Sent: Tuesday, August 15, 2023 2:42 AM
> 
> On 8/10/2023 8:25 PM, Tian, Kevin wrote:
> > Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
> >
> >
> >> 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?
> 
> Hey Kevin,
> 
> The AMD Pensando DSC provides multiple PFs for each class of VFs.
> All of our production devices will meet the assumptions of the pci core
> function above that all VFs match VF0 for those common fields.
> 
> I've been out for a few days so apologies for the delayed response.
> 

Sounds good. No more open then. 😊