[virtio-dev] [PATCH v3 0/3] transport-pci: Introduce legacy registers access using AQ

* [virtio-dev] [PATCH v3 0/3] transport-pci: Introduce legacy registers access using AQ
@ 2023-06-02 20:36 ` Parav Pandit
  0 siblings, 0 replies; 164+ messages in thread
From: Parav Pandit @ 2023-06-02 20:36 UTC (permalink / raw)
  To: mst, virtio-dev, cohuck, david.edmondson
  Cc: sburla, jasowang, yishaih, maorg, virtio-comment, shahafs, Parav Pandit

This short series introduces legacy registers access commands for the owner
group member PCI PF to access the legacy registers of the member VFs.

If in future any SIOV devices to support legacy registers, they
can be easily supported using same commands by using the group
member identifiers of the future SIOV devices.

More details as overview, motivation, use case are further described
below.

Patch summary:
--------------
patch-1 split rows of admin opcode tables by a line
patch-2 adds administrative virtuqueue commands
patch-3 adds its conformance section

This short series is on top of latest work [1] from Michael.
It uses the newly introduced administrative virtqueue facility with 3 new
commands which uses the existing virtio_admin_cmd.

[1] https://lists.oasis-open.org/archives/virtio-comment/202305/msg00112.html

Usecase:
--------
1. A hypervisor/system needs to provide transitional
   virtio devices to the guest VM at scale of thousands,
   typically, one to eight devices per VM.

2. A hypervisor/system needs to provide such devices using a
   vendor agnostic driver in the hypervisor system.

3. A hypervisor system prefers to have single stack regardless of
   virtio device type (net/blk) and be future compatible with a
   single vfio stack using SR-IOV or other scalable device
   virtualization technology to map PCI devices to the guest VM.
   (as transitional or otherwise)

Motivation/Background:
----------------------
The existing virtio transitional PCI device is missing support for
PCI SR-IOV based devices. Currently it does not work beyond
PCI PF, or as software emulated device in reality. Currently it
has below cited system level limitations:

[a] PCIe spec citation:
VFs do not support I/O Space and thus VF BARs shall not indicate I/O Space.

[b] cpu arch citiation:
Intel 64 and IA-32 Architectures Software Developer’s Manual:
The processor’s I/O address space is separate and distinct from
the physical-memory address space. The I/O address space consists
of 64K individually addressable 8-bit I/O ports, numbered 0 through FFFFH.

[c] PCIe spec citation:
If a bridge implements an I/O address range,...I/O address range will be
aligned to a 4 KB boundary.

Overview:
---------
Above usecase requirements can be solved by PCI PF group owner accessing
its group member PCI VFs legacy registers using an admin virtqueue of
the group owner PCI PF.

Two new admin virtqueue commands are added which read/write PCI VF
registers.

The third command suggested by Jason queries the VF device's driver
notification region.

Software usage example:
-----------------------
One way to use and map to the guest VM is by using vfio driver
framework in Linux kernel.

                +----------------------+
                |pci_dev_id = 0x100X   |
+---------------|pci_rev_id = 0x0      |-----+
|vfio device    |BAR0 = I/O region     |     |
|               |Other attributes      |     |
|               +----------------------+     |
|                                            |
+   +--------------+     +-----------------+ |
|   |I/O BAR to AQ |     | Other vfio      | |
|   |rd/wr mapper  |     | functionalities | |
|   +--------------+     +-----------------+ |
|                                            |
+------+-------------------------+-----------+
       |                         |
       |                    Driver notification
       |                         |
       |                         |
  +----+------------+       +----+------------+
  | +-----+         |       | PCI VF device A |
  | | AQ  |-------------+---->+-------------+ |
  | +-----+         |   |   | | legacy regs | |
  | PCI PF device   |   |   | +-------------+ |
  +-----------------+   |   +-----------------+
                        |
                        |   +----+------------+
                        |   | PCI VF device N |
                        +---->+-------------+ |
                            | | legacy regs | |
                            | +-------------+ |
                            +-----------------+

2. Virtio pci driver to bind to the listed device id and
   use it as native device in the host.

3. Use it in a light weight hypervisor to run bare-metal OS.

Please review.

Alternatives considered:
========================
1. Exposing BAR0 as MMIO BAR that follows legacy registers template
Pros:
a. Kind of works with legacy drivers as some of them have API
   which is agnostic to MMIO vs IOBAR.
b. Does not require hypervisor intervantion
Cons:
a. Device reset is extremely hard to implement in device at scale as
   driver does not wait for reset completion
b. Device register width related problems persist that hypervisor if
   wishes, cannot fix it.

2. Accessing VF registers by tunneling it through new legacy PCI capability
Pros:
a. Self contained, but cannot work with future PCI SIOV devices
Cons:
a. Equally slow as AQ access
b. Still requires new capability for notification access

conclusion for picking AQ approach:
==================================
1. Overall AQ based access is simpler to implement with combination of
   best from software and device so that legacy registers do not get baked
   in the device hardware
2. AQ allows hypervisor software to intercept legacy registers and make
   corrections if needed
3. Provides trade-off between performance, device complexity vs spec,
   while still maintaining passthrough mode for the VFs with minimal
   hypervisor intercepts only for legacy registers access
4. AQ mechanism is designed for accessing other member devices registers
   as noted in AQ submission, it utilizes the existing infrastructure over
   other alternatives.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/167
Signed-off-by: Parav Pandit <parav@nvidia.com>

---
changelog:
v2->v3:
- added new patch to split raws of admin vq opcode table
- adddressed Jason and Michael's comment to split single register
  access command to common config and device specific commands.
- dropped the suggetion to introduce enable/disable command as
  admin command cap bit already covers it.
- added other alternative design considered and discussed in detail in v0, v1 and v2

v1->v2:
- addressed comments from Michael
- added theory of operation
- grammar corrections
- removed group fields description from individual commands as
  it is already present in generic section
- added endianness normative for legacy device registers region
- renamed the file to drop vf and add legacy prefix
- added overview in commit log
- renamed subsection to reflect command

v0->v1:
- addressed comments, suggesetions and ideas from Michael Tsirkin and Jason Wang
- far more simpler design than MMR access
- removed complexities of MMR device ids
- removed complexities of MMR registers and extended capabilities
- dropped adding new extended capabilities because if if they are
  added, a pci device still needs to have existing capabilities
  in the legacy configuration space and hypervisor driver do not
  need to access them

Parav Pandit (3):
  admin: Split opcode table rows with a line
  transport-pci: Introduce legacy registers access commands
  transport-pci: Add legacy register access conformance section

 admin.tex                     |  14 ++-
 conformance.tex               |   2 +
 transport-pci-legacy-regs.tex | 189 ++++++++++++++++++++++++++++++++++
 transport-pci.tex             |   2 +
 4 files changed, 206 insertions(+), 1 deletion(-)
 create mode 100644 transport-pci-legacy-regs.tex

-- 
2.26.2

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