[virtio-dev] Re: [PATCH v2 0/2] transport-pci: Introduce legacy registers access using AQ

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, May 06, 2023 at 03:01:33AM +0300, Parav Pandit wrote:
> 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 adds administrative virtuqueue commands
> patch-2 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 | |
> |   +--------------+     +-----------------+ |
> |                                            |
> +------+-------------------------+-----------+
>        |                         |
>   +----+------------+       +----+------------+
>   | +-----+         |       | 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.
> 
> Fixes: https://github.com/oasis-tcs/virtio-spec/issues/167
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> 
> ---
> changelog:
> 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

one things I still don't see addressed here is support for
legacy interrupts. legacy driver can disable msix and
interrupts will be then sent.
how about a special command that is used when device would
normally send INT#x? it can also return ISR to reduce latency.


> 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 (2):
>   transport-pci: Introduce legacy registers access commands
>   transport-pci: Add legacy register access conformance section
> 
>  admin.tex                     |   5 +-
>  conformance.tex               |   2 +
>  transport-pci-legacy-regs.tex | 133 ++++++++++++++++++++++++++++++++++
>  transport-pci.tex             |   2 +
>  4 files changed, 141 insertions(+), 1 deletion(-)
>  create mode 100644 transport-pci-legacy-regs.tex
> 
> -- 
> 2.26.2


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