[RFC PATCH 0/3] Introduce a PCIe endpoint virtio console

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PCIe endpoint framework provides APIs to implement PCIe endpoint function.
This framework allows defining various PCIe endpoint function behaviors in
software. This patch extend the framework for virtio pci device. The
virtio is defined to communicate guest on virtual machine and host side.
Advantage of the virtio is the efficiency of data transfer and the conciseness
of implementation device using software. It also be applied to PCIe
endpoint function.

We designed and implemented a PCIe EP virtio console function driver using
the extended PCIe endpoint framework for virtio. It can be communicate
host and endpoint over virtio as console.

An architecture of the function driver is following:

 ┌────────────┐         ┌──────────────────────┬────────────┐
 │virtio      │         │                      │virtio      │
 │console drv │         ├───────────────┐      │console drv │
 ├────────────┤         │(virtio console│      ├────────────┤
 │ virtio bus │         │ device)       │◄────►│ virtio bus │
 ├────────────┤         ├---------------┤      └────────────┤
 │            │         │ pci ep virtio │                   │
 │  pci bus   │         │  console drv  │                   │
 │            │  pcie   ├───────────────┤                   │
 │            │ ◄─────► │  pci ep Bus   │                   │
 └────────────┘         └───────────────┴───────────────────┘
   PCIe Root              PCIe Endpoint

Introduced driver is `pci ep virtio console drv` in the figure. It works
as ep function for PCIe root and virtual virtio console device for PCIe
endpoint. Each side of virtio console driver has virtqueue, and
introduced driver transfers data on the virtqueue to each other. A data
on root tx queue is transfered to endpoint rx queue and vice versa.

This patchset is depend follwing patches which are under discussion.

- [RFC PATCH 0/3] Deal with alignment restriction on EP side
link: https://lore.kernel.org/linux-pci/20230113090350.1103494-1-mie@xxxxxxxxxx/
- [RFC PATCH v2 0/7] Introduce a vringh accessor for IO memory
link: https://lore.kernel.org/virtualization/20230202090934.549556-1-mie@xxxxxxxxxx/

First of this patchset is introduce a helper function to realize pci
virtio function using PCIe endpoint framework. The second one is adding
a missing definition for virtio pci header. The last one is for PCIe
endpoint virtio console driver.

This is tested on linux-20230406 and RCar S4 board as PCIe endpoint.

Shunsuke Mie (3):
  PCI: endpoint: introduce a helper to implement pci ep virtio function
  virtio_pci: add a definition of queue flag in ISR
  PCI: endpoint: Add EP function driver to provide virtio-console

 drivers/pci/endpoint/functions/Kconfig        |  19 +
 drivers/pci/endpoint/functions/Makefile       |   2 +
 drivers/pci/endpoint/functions/pci-epf-vcon.c | 554 ++++++++++++++++++
 .../pci/endpoint/functions/pci-epf-virtio.c   | 469 +++++++++++++++
 .../pci/endpoint/functions/pci-epf-virtio.h   | 123 ++++
 include/uapi/linux/virtio_pci.h               |   3 +
 6 files changed, 1170 insertions(+)
 create mode 100644 drivers/pci/endpoint/functions/pci-epf-vcon.c
 create mode 100644 drivers/pci/endpoint/functions/pci-epf-virtio.c
 create mode 100644 drivers/pci/endpoint/functions/pci-epf-virtio.h


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