Re: [PATCH 3/3] Documentation: fpga: dfl: Add description for VFIO Mdev support

[Alex Williamson]

On Tue,  8 Sep 2020 15:13:32 +0800
Xu Yilun <yilun.xu@xxxxxxxxx> wrote:

> This patch adds description for VFIO Mdev support for dfl devices on
> dfl bus.
> 
> Signed-off-by: Xu Yilun <yilun.xu@xxxxxxxxx>
> Signed-off-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
> ---
>  Documentation/fpga/dfl.rst | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst
> index 0404fe6..f077754 100644
> --- a/Documentation/fpga/dfl.rst
> +++ b/Documentation/fpga/dfl.rst
> @@ -502,6 +502,26 @@ FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c)
>  could be a reference.
>  
>  
> +VFIO Mdev support for DFL devices
> +=================================
> +As we introduced a dfl bus for private features, they could be added to dfl bus
> +as independent dfl devices. There is a requirement to handle these devices
> +either by kernel drivers or by direct access from userspace. Usually we bind
> +the kernel drivers to devices which provide board management functions, and
> +gives user direct access to devices which cooperate closely with user
> +controlled Accelerated Function Unit (AFU). We realize this with a VFIO Mdev
> +implementation. When we bind the vfio-mdev-dfl driver to a dfl device, it
> +realizes a group of callbacks and registers to the Mdev framework as a
> +parent (physical) device. It could then create one (available_instances == 1)
> +mdev device.
> +Since dfl devices are sub devices of FPGA DFL physical devices (e.g. PCIE
> +device), which provide no DMA isolation for each sub device, this may leads to
> +DMA isolation problem if a private feature is designed to be capable of DMA.
> +The AFU user could potentially access the whole device addressing space and
> +impact the private feature. So now the general HW design rule is, no DMA
> +capability for private features. It eliminates the DMA isolation problem.

What's the advantage of entangling mdev/vfio in this approach versus
simply exposing the MMIO region of the device via sysfs (similar to a
resource file in pci-sysfs)?  This implementation doesn't support
interrupts, it doesn't support multiplexing of a device, it doesn't
perform any degree of mediation, it seems to simply say "please don't
do DMA".  I don't think that's acceptable for an mdev driver.  If you
want to play loose with isolation, do it somewhere else.  Thanks,

Alex