Re: [RFCv2 PATCH 0/7] A General Accelerator Framework, WarpDrive

Jerome Glisse
2018-09-04T15:00:28Z

On Mon, Sep 03, 2018 at 08:51:57AM +0800, Kenneth Lee wrote:
> From: Kenneth Lee <liguozhu@hisilicon.com>
>
> WarpDrive is an accelerator framework to expose the hardware capabilities
> directly to the user space. It makes use of the exist vfio and vfio-mdev
> facilities. So the user application can send request and DMA to the
> hardware without interaction with the kernel. This removes the latency
> of syscall.
>
> WarpDrive is the name for the whole framework. The component in kernel
> is called SDMDEV, Share Domain Mediated Device. Driver driver exposes its
> hardware resource by registering to SDMDEV as a VFIO-Mdev. So the user
> library of WarpDrive can access it via VFIO interface.
>
> The patchset contains document for the detail. Please refer to it for more
> information.
>
> This patchset is intended to be used with Jean Philippe Brucker's SVA
> patch [1], which enables not only IO side page fault, but also PASID
> support to IOMMU and VFIO.
>
> With these features, WarpDrive can support non-pinned memory and
> multi-process in the same accelerator device. We tested it in our SoC
> integrated Accelerator (board ID: D06, Chip ID: HIP08). A reference work
> tree can be found here: [2].
>
> But it is not mandatory. This patchset is tested in the latest mainline
> kernel without the SVA patches. So it supports only one process for each
> accelerator.
>
> We have noticed the IOMMU aware mdev RFC announced recently [3].
>
> The IOMMU aware mdev has similar idea but different intention comparing to
> WarpDrive. It intends to dedicate part of the hardware resource to a VM.
> And the design is supposed to be used with Scalable I/O Virtualization.
> While sdmdev is intended to share the hardware resource with a big amount
> of processes. It just requires the hardware supporting address
> translation per process (PCIE's PASID or ARM SMMU's substream ID).
>
> But we don't see serious confliction on both design. We believe they can be
> normalized as one.
>

So once again i do not understand why you are trying to do things
this way. Kernel already have tons of example of everything you
want to do without a new framework. Moreover i believe you are
confuse by VFIO. To me VFIO is for VM not to create general device
driver frame work.

So here is your use case as i understand it. You have a device
with a limited number of command queues (can be just one) and in
some case it can support SVA/SVM (when hardware support it and it
is not disabled). Final requirement is being able to schedule cmds
from userspace without ioctl. All of this exists already exists
upstream in few device drivers.

So here is how every body else is doing it. Please explain why
this does not work.

1 Userspace open device file driver. Kernel device driver create
a context and associate it with on open. This context can be
uniq to the process and can bind hardware resources (like a
command queue) to the process.
2 Userspace bind/acquire a commands queue and initialize it with
an ioctl on the device file. Through that ioctl userspace can
be inform wether either SVA/SVM works for the device. If SVA/
SVM works then kernel device driver bind the process to the
device as part of this ioctl.
3 If SVM/SVA does not work userspace do an ioctl to create dma
buffer or something that does exactly the same thing.
4 Userspace mmap the command queue (mmap of the device file by
using informations gather at step 2)
5 Userspace can write commands into the queue it mapped
6 When userspace close the device file all resources are release
just like any existing device drivers.

Now if you want to create a device driver framework that expose
a device file with generic API for all of the above steps fine.
But it does not need to be part of VFIO whatsoever or explain
why.

Note that if IOMMU is fully disabled you probably want to block
userspace from being able to directly scheduling commands onto
the hardware as it would allow userspace to DMA anywhere and thus
would open the kernel to easy exploits. In this case you can still
keeps the same API as above and use page fault tricks to valid
commands written by userspace into fake commands ring. This will
be as slow or maybe even slower than ioctl but at least it allows
you to validate commands.

Cheers,
Jérôme