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Date:   Mon, 18 Jan 2021 14:38:06 +0100
From:   Cornelia Huck <cohuck@redhat.com>
To:     Max Gurtovoy <mgurtovoy@nvidia.com>
Cc:     <alex.williamson@redhat.com>, <kvm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <jgg@nvidia.com>,
        <liranl@nvidia.com>, <oren@nvidia.com>, <tzahio@nvidia.com>,
        <leonro@nvidia.com>, <yarong@nvidia.com>, <aviadye@nvidia.com>,
        <shahafs@nvidia.com>, <artemp@nvidia.com>, <kwankhede@nvidia.com>,
        <ACurrid@nvidia.com>, <gmataev@nvidia.com>, <cjia@nvidia.com>
Subject: Re: [PATCH RFC v1 0/3] Introduce vfio-pci-core subsystem
Message-ID: <20210118143806.036c8dbc.cohuck@redhat.com>
In-Reply-To: <20210117181534.65724-1-mgurtovoy@nvidia.com>
References: <20210117181534.65724-1-mgurtovoy@nvidia.com>
Organization: Red Hat GmbH
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On Sun, 17 Jan 2021 18:15:31 +0000
Max Gurtovoy <mgurtovoy@nvidia.com> wrote:

> Hi Alex and Cornelia,
> 
> This series split the vfio_pci driver into 2 parts: pci driver and a
> subsystem driver that will also be library of code. The pci driver,
> vfio_pci.ko will be used as before and it will bind to the subsystem
> driver vfio_pci_core.ko to register to the VFIO subsystem. This patchset
> if fully backward compatible. This is a typical Linux subsystem
> framework behaviour. This framework can be also adopted by vfio_mdev
> devices as we'll see in the below sketch.
> 
> This series is coming to solve the issues that were raised in the
> previous attempt for extending vfio-pci for vendor specific
> functionality: https://lkml.org/lkml/2020/5/17/376 by Yan Zhao.
> 
> This solution is also deterministic in a sense that when a user will
> bind to a vendor specific vfio-pci driver, it will get all the special
> goodies of the HW.
>  
> This subsystem framework will also ease on adding vendor specific
> functionality to VFIO devices in the future by allowing another module
> to provide the pci_driver that can setup number of details before
> registering to VFIO subsystem (such as inject its own operations).
> 
> Below we can see the proposed changes (this patchset only deals with
> VFIO_PCI subsystem but it can easily be extended to VFIO_MDEV subsystem
> as well):
> 
> +----------------------------------------------------------------------+
> |                                                                      |
> |                                VFIO                                  |
> |                                                                      |
> +----------------------------------------------------------------------+
> 
> +--------------------------------+    +--------------------------------+
> |                                |    |                                |
> |          VFIO_PCI_CORE         |    |          VFIO_MDEV_CORE        |
> |                                |    |                                |
> +--------------------------------+    +--------------------------------+
> 
> +---------------+ +--------------+    +---------------+ +--------------+
> |               | |              |    |               | |              |
> |               | |              |    |               | |              |
> | VFIO_PCI      | | MLX5_VFIO_PCI|    | VFIO_MDEV     | |MLX5_VFIO_MDEV|
> |               | |              |    |               | |              |
> |               | |              |    |               | |              |
> +---------------+ +--------------+    +---------------+ +--------------+
> 
> First 2 patches introduce the above changes for vfio_pci and
> vfio_pci_core.
> 
> Patch (3/3) introduces a new mlx5 vfio-pci module that registers to VFIO
> subsystem using vfio_pci_core. It also registers to Auxiliary bus for
> binding to mlx5_core that is the parent of mlx5-vfio-pci devices. This
> will allow extending mlx5-vfio-pci devices with HW specific features
> such as Live Migration (mlx5_core patches are not part of this series
> that comes for proposing the changes need for the vfio pci subsystem).
> 
> These devices will be seen on the Auxiliary bus as:
> mlx5_core.vfio_pci.2048 -> ../../../devices/pci0000:00/0000:00:02.0/0000:05:00.0/0000:06:00.0/0000:07:00.0/mlx5_core.vfio_pci.2048
> mlx5_core.vfio_pci.2304 -> ../../../devices/pci0000:00/0000:00:02.0/0000:05:00.0/0000:06:00.0/0000:07:00.1/mlx5_core.vfio_pci.2304
> 
> 2048 represents BDF 08:00.0 and 2304 represents BDF 09:00.0 in decimal
> view. In this manner, the administrator will be able to locate the
> correct vfio-pci module it should bind the desired BDF to (by finding
> the pointer to the module according to the Auxiliary driver of that
> BDF).

I'm not familiar with that auxiliary framework (it seems to be fairly
new?); but can you maybe create an auxiliary device unconditionally and
contain all hardware-specific things inside a driver for it? Or is that
not flexible enough?

> 
> In this way, we'll use the HW vendor driver core to manage the lifecycle
> of these devices. This is reasonable since only the vendor driver knows
> exactly about the status on its internal state and the capabilities of
> its acceleratots, for example.
> 
> TODOs:
> 1. For this RFC we still haven't cleaned all vendor specific stuff that
>    were merged in the past into vfio_pci (such as VFIO_PCI_IG and
>    VFIO_PCI_NVLINK2).
> 2. Create subsystem module for VFIO_MDEV. This can be used for vendor
>    specific scalable functions for example (SFs).
> 3. Add Live migration functionality for mlx5 SNAP devices
>    (NVMe/Virtio-BLK).
> 4. Add Live migration functionality for mlx5 VFs
> 5. Add the needed functionality for mlx5_core
> 
> I would like to thank the great team that was involved in this
> development, design and internal review:
> Oren, Liran, Jason, Leon, Aviad, Shahaf, Gary, Artem, Kirti, Neo, Andy
> and others.
> 
> This series applies cleanly on top of kernel 5.11-rc2+ commit 2ff90100ace8:
> "Merge tag 'hwmon-for-v5.11-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging"
> from Linus.
> 
> Note: Live migration for MLX5 SNAP devices is WIP and will be the first
>       example for adding vendor extension to vfio-pci devices. As the
>       changes to the subsystem must be defined as a pre-condition for
>       this work, we've decided to split the submission for now.
> 
> Max Gurtovoy (3):
>   vfio-pci: rename vfio_pci.c to vfio_pci_core.c
>   vfio-pci: introduce vfio_pci_core subsystem driver
>   mlx5-vfio-pci: add new vfio_pci driver for mlx5 devices
> 
>  drivers/vfio/pci/Kconfig            |   22 +-
>  drivers/vfio/pci/Makefile           |   16 +-
>  drivers/vfio/pci/mlx5_vfio_pci.c    |  253 +++
>  drivers/vfio/pci/vfio_pci.c         | 2386 +--------------------------
>  drivers/vfio/pci/vfio_pci_core.c    | 2311 ++++++++++++++++++++++++++

Especially regarding this diffstat... from a quick glance at patch 3,
it mostly forwards to vfio_pci_core anyway. Do you expect a huge amount
of device-specific callback invocations?

[I have not looked at this in detail yet.]

>  drivers/vfio/pci/vfio_pci_private.h |   21 +
>  include/linux/mlx5/vfio_pci.h       |   36 +
>  7 files changed, 2734 insertions(+), 2311 deletions(-)
>  create mode 100644 drivers/vfio/pci/mlx5_vfio_pci.c
>  create mode 100644 drivers/vfio/pci/vfio_pci_core.c
>  create mode 100644 include/linux/mlx5/vfio_pci.h
>