[virtio-dev] [PATCH v12 0/3] admin: Access legacy registers using admin commands

Parav Pandit
2023-07-07T03:54:38Z

This short series introduces legacy registers access commands for the owner
group member access the legacy registers of the member VFs.
This short series introduces legacy region access commands by the group owner
device for its member devices.
Currently it is applicable to the PCI PF and VF devices. 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 fix split rows of admin opcode tables by a line
patch-2 fix section numbering
patch-3 add legacy region access commands

It uses the newly introduced administration command facility with 4 new
commands and a new optional command to query the legacy notification region.

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 is 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.

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 | |
| +--------------+ +-----------------+ |
| |
+------+-------------------------+-----------+
| |
Legacy region Driver notification
access |
| |
+----+------------+ +----+------------+
| +-----+ | | 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 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 used 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 device reset completion
b. Device register width related problems persist that hypervisor if
wishes, it cannot be fixed.

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
c. Requires hardware to build low level registers access which is not worth
for long term future

3. Accessing VF notification region using new PF BAR
Cons:
a. Requires hardware to build new PCI steering logic per PF to forward
notification from the PF to VF, requires double the amount of logic
compared to today
b. Requires very large additional PF BAR whose size must be max_Vfs * BAR size.

4. Trapping CVQ, configuration region, LEGACY_HDR
Cons:
a. This does not fullfil the very basic requirement to not trap the
1.x objects (configuration registers, vqs)
b. Requires feature negotiations mediation in hypervisor software
c. Requires constant device type specific knowledge in hypervisor driver
(Does not scale for 30+ device types)

4. F_LEACY_HDR, F_WRITE_MAC
Cons:
a. Requires device support to have read/write mac address which is
hard to implement on every member device.
b. such functionality is duplicate of existing cvq per device.
c. config space is only for the initialization specific purpose.
d. Requires mediation of 1.x objects, which is not good design.
e. Solves only for the net device.
Pros:
a. May work for nested env

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.
5. Uses existing driver notification region similar to legacy notification
saves hardware resources

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

---
changelog:
v11->v12:
- added missing article the at few places
- rewrote group_member_id statements like other existing
commands which is cleaner and shorter
- added length and alignment lines to multiple commands
- rewrote fast path to separate dedicated mechanism
- rewrote example and description para for legacy notification command
- made separate paragraph for the notify info command
- dropped citation to virtio pci capabilities for member device
- notification region changed to notification address throughout
- added description to all the fields of the info struct
- avoided union in spirit of keeping all for pci
- used single listing
- moved description to end which was in between two structs
- added 4 entry and preference description
- added conformance line for notification via mmio works same way as
admin command
v10->v11:
- replaced tab with white spaces in read structure
- included pci fields along side other generic fields to avoid
indirection
- merged pci conformance section
- avoid using definite in starting introduction
- replace 'all of the' with 'any of the'
- changed drivers notification normative to indicate use of
NOTIFY_INFO command
- renamed NOTIFY_QUERY to NOTIFY_INFO name
- merged 4th patch with 3rd
- added normative line for notify_info command
- reworded notification region command description to be more verbose
- merged flags and owner field to indicate end of list
v9->v10:
- added white space at end of line
- addressed below comments from Cornelia
- fixed errors related to article
- hardwire to hardwires
- replaced various to all
- added hardwire to zero
- fixed requirements for administration virtqueue section
- added missing articles
- reworded description for notification query command
- grammar fixes
- addressed below comments from Michael
- added description for member group id setting
- reworded device and driver conformance statements
- opcode table description updated
- fixed label for device read command
- length alignment restriction text added
- data length described for read write commands
- notification description added and refined
- reworded text around command specific result and data field usage
v8->v9:
- add missing articles in notify query command
- replaced 'this notification' with 'such a notification'
- addressed below comments from Michael
- dropped 'Region' from the commands
- added 7 reserved pad bytes in config write commands
- rewrote from 'use following structure' to 'field' has the following
struct..
- dropped mentioning to follow struct virtio_admin_cmd.
- added note about command limited to only sriov group type for now
- rewrote the description little differently
v7->v8:
- remove empty line at the end of file
- removed white space at the end
- addressed comments from Michael add link to pci
- renamed region to region_data
- made region_data width to be 16 bytes to cover for 8 bytes offset
- moved generic notification region related normative from pci to
generic section
- addressed comments from Michael
- made bar offset 64-bit
- prefix legacy specific structure with _legacy
- moved generic normative from pci to generic section
- added link to virtio pci capabilities when referring to bar 0
- remove 'should' from generic description
v6->v7:
- addressed several comments from Michael
- use AQ command to query legacy notify region, dropped pci capability
modifications
- moved most part of the text to the generic admin command section
- replace administrative to administration
- replace admin vq citation to admin commands
- added normatives for device and driver side
- made BAR0 to be not used at all when supporting legacy interface
- added normative around BAR0 and SR-IOV extended capability
- grammar corrections
v5->v6:
- fixed previous missed abbreviation of LCC and LD
- added text for the PCI capability for the group member device
v4->v5:
- split pci transport and generic command section to new patch
- removed multiple references to the VF
- written the description of the command as generic with member
and group device terminology
- reflected many section names to remove VF
- split from pci transport specific patch
- split conformance to transport and generic sections
- written the description of the command as generic with member
and group device terminology
- reflected many section names to remove VF
- rename fields from register to region
- avoided abbreviation for legacy, device and config
v3->v4:
- moved noted to the conformance section details in next patch
- removed queue notify address query AQ command on Michael's suggestion,
though it is fine. Instead replaced with extending virtio_pci_notify_cap
to indicate that legacy queue notifications can be done on the
notification location
- fixed spelling errors
- replaced administrative virtqueue to administration virtqueue
- moved legacy interface normative references to legacy conformance
section
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
admin: Fix section numbering
admin: Add group member legacy register access commands

admin-cmds-legacy-interface.tex | 329 ++++++++++++++++++++++++++++++++
admin.tex | 24 ++-
conformance.tex | 2 +
3 files changed, 350 insertions(+), 5 deletions(-)
create mode 100644 admin-cmds-legacy-interface.tex

--
2.26.2