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SubjectRe: [RFC PATCH 0/7] A General Accelerator Framework, WarpDrive
On Thu, Aug 09, 2018 at 10:46:13AM -0400, Jerome Glisse wrote:
> Date: Thu, 9 Aug 2018 10:46:13 -0400
> From: Jerome Glisse <jglisse@redhat.com>
> To: Kenneth Lee <liguozhu@hisilicon.com>
> CC: Kenneth Lee <nek.in.cn@gmail.com>, "Tian, Kevin"
> <kevin.tian@intel.com>, Alex Williamson <alex.williamson@redhat.com>,
> Herbert Xu <herbert@gondor.apana.org.au>, "kvm@vger.kernel.org"
> <kvm@vger.kernel.org>, Jonathan Corbet <corbet@lwn.net>, Greg
> Kroah-Hartman <gregkh@linuxfoundation.org>, Zaibo Xu <xuzaibo@huawei.com>,
> "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>, "Kumar, Sanjay K"
> <sanjay.k.kumar@intel.com>, Hao Fang <fanghao11@huawei.com>,
> "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
> "linuxarm@huawei.com" <linuxarm@huawei.com>,
> "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
> "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>, Philippe
> Ombredanne <pombredanne@nexb.com>, Thomas Gleixner <tglx@linutronix.de>,
> "David S . Miller" <davem@davemloft.net>,
> "linux-accelerators@lists.ozlabs.org"
> <linux-accelerators@lists.ozlabs.org>
> Subject: Re: [RFC PATCH 0/7] A General Accelerator Framework, WarpDrive
> User-Agent: Mutt/1.10.0 (2018-05-17)
> Message-ID: <20180809144613.GB3386@redhat.com>
>
> On Thu, Aug 09, 2018 at 04:03:52PM +0800, Kenneth Lee wrote:
> > On Wed, Aug 08, 2018 at 11:18:35AM -0400, Jerome Glisse wrote:
> > > On Wed, Aug 08, 2018 at 09:08:42AM +0800, Kenneth Lee wrote:
> > > > 在 2018年08月06日 星期一 11:32 下午, Jerome Glisse 写道:
> > > > > On Mon, Aug 06, 2018 at 11:12:52AM +0800, Kenneth Lee wrote:
> > > > > > On Fri, Aug 03, 2018 at 10:39:44AM -0400, Jerome Glisse wrote:
> > > > > > > On Fri, Aug 03, 2018 at 11:47:21AM +0800, Kenneth Lee wrote:
> > > > > > > > On Thu, Aug 02, 2018 at 10:22:43AM -0400, Jerome Glisse wrote:
> > > > > > > > > On Thu, Aug 02, 2018 at 12:05:57PM +0800, Kenneth Lee wrote:
>
> [...]
>
> > > > > > > > > your mechanisms the userspace must have a specific userspace
> > > > > > > > > drivers for each hardware and thus there are virtually no
> > > > > > > > > differences between having this userspace driver open a device
> > > > > > > > > file in vfio or somewhere else in the device filesystem. This is
> > > > > > > > > just a different path.
> > > > > > > > >
> > > > > > > > The basic problem WarpDrive want to solve it to avoid syscall. This is important
> > > > > > > > to accelerators. We have some data here:
> > > > > > > > https://www.slideshare.net/linaroorg/progress-and-demonstration-of-wrapdrive-a-accelerator-framework-sfo17317
> > > > > > > >
> > > > > > > > (see page 3)
> > > > > > > >
> > > > > > > > The performance is different on using kernel and user drivers.
> > > > > > > Yes and example i point to is exactly that. You have a one time setup
> > > > > > > cost (creating command buffer binding PASID with command buffer and
> > > > > > > couple other setup steps). Then userspace no longer have to do any
> > > > > > > ioctl to schedule work on the GPU. It is all down from userspace and
> > > > > > > it use a doorbell to notify hardware when it should go look at command
> > > > > > > buffer for new thing to execute.
> > > > > > >
> > > > > > > My point stands on that. You have existing driver already doing so
> > > > > > > with no new framework and in your scheme you need a userspace driver.
> > > > > > > So i do not see the value add, using one path or the other in the
> > > > > > > userspace driver is litteraly one line to change.
> > > > > > >
> > > > > > Sorry, I'd got confuse here. I partially agree that the user driver is
> > > > > > redundance of kernel driver. (But for WarpDrive, the kernel driver is a full
> > > > > > driver include all preparation and setup stuff for the hardware, the user driver
> > > > > > is simply to send request and receive answer). Yes, it is just a choice of path.
> > > > > > But the user path is faster if the request come from use space. And to do that,
> > > > > > we need user land DMA support. Then why is it invaluable to let VFIO involved?
> > > > > Some drivers in the kernel already do exactly what you said. The user
> > > > > space emit commands without ever going into kernel by directly scheduling
> > > > > commands and ringing a doorbell. They do not need VFIO either and they
> > > > > can map userspace address into the DMA address space of the device and
> > > > > again they do not need VFIO for that.
> > > > Could you please directly point out which driver you refer to here? Thank
> > > > you.
> > >
> > > drivers/gpu/drm/amd/
> > >
> > > Sub-directory of interest is amdkfd
> > >
> > > Because it is a big driver here is a highlevel overview of how it works
> > > (this is a simplification):
> > > - Process can allocate GPUs buffer (through ioclt) and map them into
> > > its address space (through mmap of device file at buffer object
> > > specific offset).
> > > - Process can map any valid range of virtual address space into device
> > > address space (IOMMU mapping). This must be regular memory ie not an
> > > mmap of a device file or any special file (this is the non PASID
> > > path)
> > > - Process can create a command queue and bind its process to it aka
> > > PASID, this is done through an ioctl.
> > > - Process can schedule commands onto queues it created from userspace
> > > without ioctl. For that it just write command into a ring buffer
> > > that it mapped during the command queue creation process and it
> > > rings a doorbell when commands are ready to be consume by the
> > > hardware.
> > > - Commands can reference (access) all 3 types of object above ie
> > > either full GPUs buffer, process regular memory maped as object
> > > (non PASID) and PASID memory all at the same time ie you can
> > > mix all of the above in same commands queue.
> > > - Kernel can evict, unbind any process command queues, unbind commands
> > > queue are still valid from process point of view but commands
> > > process schedules on them will not be executed until kernel re-bind
> > > the queue.
> > > - Kernel can schedule commands itself onto its dedicated command
> > > queues (kernel driver create its own command queues).
> > > - Kernel can control priorities between all the queues ie it can
> > > decides which queues should the hardware executed first next.
> > >
> >
> > Thank you. Now I think I understand the point. Indeed, I can see some drivers,
> > such GPU and IB, attach their own iommu_domain to their iommu_group and do their
> > own iommu_map().
> >
> > But we have another requirement which is to combine some device together to
> > share the same address space. This is a little like these kinds of solution:
> >
> > http://tce.technion.ac.il/wp-content/uploads/sites/8/2015/06/SC-7.2-M.-Silberstein.pdf
> >
> > With that, the application can directly pass the NiC packet pointer to the
> > decryption accelerator, and get the bare data in place. This is the feature that
> > the VFIO container can provide.
>
> Yes and GPU would very much like do the same. There is already out of
> tree solution that allow NiC to stream into GPU memory or GPU to stream
> its memory to a NiC. I am sure we will want to use more accelerator in
> conjunction with GPU in the future.
>
> >
> > > I believe all of the above are the aspects that matters to you. The main
> > > reason i don't like creating a new driver infrastructure is that a lot
> > > of existing drivers will want to use some of the new features that are
> > > coming (memory topology, where to place process memory, pipeline devices,
> > > ...) and thus existing drivers are big (GPU drivers are the biggest of
> > > all the kernel drivers).
> > >
> >
> > I think it is not necessarily to rewrite the GPU driver if they don't need to
> > share their space with others. But if they do, no matter how, they have to create
> > some facility similar to VFIO container. Then why not just create them in VFIO?
>
> No they do not, nor does anyone needs to. We already have that. If you want
> device to share memory object you have either:
> - PASID and everything is just easy no need to create anything, as
> all valid virtual address will work
> - no PASID or one of the device does not support PASID then use the
> existing kernel infrastructure aka dma buffer see Documentation/
> driver-api/dma-buf.rst
>
> Everything you want to do is already happening upstream and they are allready
> working example.

OK, I accept it.

>
> > Actually, some GPUs have already used mdev to manage the resource by different
> > users, it is already part of VFIO.
>
> The current use of mdev with GPU is to "emulate" the SR_IOV of PCIE in
> software so that a single device can be share between multiple guests.
> For this using VFIO make sense, as we want to expose device as a single
> entity that can be manage without the userspace (QEMU) having to know
> or learn about each individual devices.
>
> QEMU just has a well define API to probe and attach device to guest.
> It is the guest that have dedicated drivers for each of those mdev
> devices.
>

Agree

>
> What you are trying to do is recreate a whole driver API inside the
> VFIO subsystem and i do not see any valid reasons for that. Moreover
> you want to restrict future use to only drivers that are part of this
> new driver subsystem and again i do not see any good reasons to
> mandate any of the existing driver to be rewritten inside a new VFIO
> infrastructure.
>

I think you have persuaded me now. I thought VFIO was the only unified place to do
user land DMA operations. So I tried to make use of it even there was facility I
needed not.

Thank you very much for the help.

>
> You can achieve everything you want to achieve with existing upstream
> solution. Re-inventing a whole new driver infrastructure should really
> be motivated with strong and obvious reasons.

I want to understand better of your idea. If I create some unified helper
APIs in drivers/iommu/, say:

wd_create_dev(parent_dev, wd_dev)
wd_release_dev(wd_dev)

The API create chrdev to take request from user space for open(resource
allocation), iomap, epoll (irq), and dma_map(with pasid automatically).

Do you think it is acceptable?

I agree that these all can be done with the driver itself. But without a unified
interface, it is hard to create a user land ecosystem for accelerator to
cooperate.

>
> Cheers,
> Jérôme

--
-Kenneth(Hisilicon)

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