Re: [PATCH RFC 10/11] nvmet: Add addr fam and trtype for mdev pci driver

[Christoph Hellwig <hch@lst.de>]

On Thu, Mar 13, 2025 at 12:18:11AM -0500, Mike Christie wrote:
> This allocates 253 for mdev pci since it might not fit into any
> existing value (not sure how to co-exist with pci-epf).
> 
> One of the reasons this patchset is a RFC is because I was not sure
> if allocating a new number for this was the best. Another approach
> is that I could break up pci-epf into a:
> 
> 1. PCI component - Common PCI and NVMe PCI code.
> 2. Interface/bus component - Callouts so pci-epf can use the
> pci_epf_driver/pci_epf_ops and mdev-pci can use mdev and vfio
> callouts.
> 3. Memory management component - Callouts for using DMA for pci-epf
> vs vfio related memory for mdev-pci.
> 
> On one hand, by creating a core nvmet pci driver then have subdrivers
> we could share NVMF_ADDR_FAMILY_PCI and NVMF_TRTYPE_PCI. However,
> it will get messy. There is some PCI code we could share for 1
> but 2 and 3 will make sharing difficult becuse of how different the
> drivers work (mdev-vfio vs pci-epf layers).

I think we'll need to discuss this more based on concrete code proposals
once we go along, but here's my handwavy 2cents for now:

  - in addition to the pure software endpoint and mdev I also expect
    hardardware offloaded PCIe endpoints to show up really soon, so
    we'll have more than just the two
  - having common code for different PCIe tagets where applicable is
    thus a good idea, but I'd expect it to be a set of library
    functions or conditionals in the core code, not a new layer
    with indirect calls
  - I had quite a lot of discussions with Damien about the trtype and
    related bits.  I suspect by the time we get to having multiple
    PCIe endpoints we just need to split the configfs interface naming
    from the on-wire fabrics trtrype enum to not need trtype assignments.