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Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

On 04/20/2018 10:19 AM, Daniel Vetter wrote:
On Wed, Apr 18, 2018 at 11:10:58AM +0100, Roger Pau Monné wrote:
On Wed, Apr 18, 2018 at 11:01:12AM +0300, Oleksandr Andrushchenko wrote:
On 04/18/2018 10:35 AM, Roger Pau Monné wrote:
On Wed, Apr 18, 2018 at 09:38:39AM +0300, Oleksandr Andrushchenko wrote:
On 04/17/2018 11:57 PM, Dongwon Kim wrote:
On Tue, Apr 17, 2018 at 09:59:28AM +0200, Daniel Vetter wrote:
On Mon, Apr 16, 2018 at 12:29:05PM -0700, Dongwon Kim wrote:
3.2 Backend exports dma-buf to xen-front

In this case Dom0 pages are shared with DomU. As before, DomU can only write
to these pages, not any other page from Dom0, so it can be still considered
But, the following must be considered (highlighted in xen-front's Kernel
   - If guest domain dies then pages/grants received from the backend cannot
     be claimed back - think of it as memory lost to Dom0 (won't be used for
     other guest)
   - Misbehaving guest may send too many requests to the backend exhausting
     its grant references and memory (consider this from security POV). As the
     backend runs in the trusted domain we also assume that it is trusted as
     e.g. must take measures to prevent DDoS attacks.
I cannot parse the above sentence:

"As the backend runs in the trusted domain we also assume that it is
trusted as well, e.g. must take measures to prevent DDoS attacks."

What's the relation between being trusted and protecting from DoS
I mean that we trust the backend that it can prevent Dom0
from crashing in case DomU's frontend misbehaves, e.g.
if the frontend sends too many memory requests etc.
In any case, all? PV protocols are implemented with the frontend
sharing pages to the backend, and I think there's a reason why this
model is used, and it should continue to be used.
This is the first use-case above. But there are real-world
use-cases (embedded in my case) when physically contiguous memory
needs to be shared, one of the possible ways to achieve this is
to share contiguous memory from Dom0 to DomU (the second use-case above)
Having to add logic in the backend to prevent such attacks means

   - We need more code in the backend, which increases complexity and
     chances of bugs.
   - Such code/logic could be wrong, thus allowing DoS.
You can live without this code at all, but this is then up to
backend which may make Dom0 down because of DomU's frontend doing evil
IMO we should design protocols that do not allow such attacks instead
of having to defend against them.

4. xen-front/backend/xen-zcopy synchronization

4.1. As I already said in 2) all the inter VM communication happens between
xen-front and the backend, xen-zcopy is NOT involved in that.
When xen-front wants to destroy a display buffer (dumb/dma-buf) it issues a
This call is synchronous, so xen-front expects that backend does free the
buffer pages on return.

    - closes all dumb handles/fd's of the buffer according to [3]
    - issues DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE IOCTL to xen-zcopy to make
      the buffer is freed (think of it as it waits for dma-buf->release
So this zcopy thing keeps some kind of track of the memory usage? Why
can't the user-space backend keep track of the buffer usage?
Because there is no dma-buf UAPI which allows to track the buffer life cycle
(e.g. wait until dma-buf's .release callback is called)
    - replies to xen-front that the buffer can be destroyed.
This way deletion of the buffer happens synchronously on both Dom0 and DomU
sides. In case if DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE returns with time-out
(BTW, wait time is a parameter of this IOCTL), Xen will defer grant
removal and will retry later until those are free.

Hope this helps understand how buffers are synchronously deleted in case
of xen-zcopy with a single protocol command.

I think the above logic can also be re-used by the hyper-dmabuf driver with
some additional work:

1. xen-zcopy can be split into 2 parts and extend:
1.1. Xen gntdev driver [4], [5] to allow creating dma-buf from grefs and
vise versa,
I don't know much about the dma-buf implementation in Linux, but
gntdev is a user-space device, and AFAICT user-space applications
don't have any notion of dma buffers. How are such buffers useful for
user-space? Why can't this just be called memory?
A dma-buf is seen by user-space as a file descriptor and you can
pass it to different drivers then. For example, you can share a buffer
used by a display driver for scanout with a GPU, to compose a picture
into it:
1. User-space (US) allocates a display buffer from display driver
2. US asks display driver to export the dma-buf which backs up that buffer,
US gets buffer's fd: dma_buf_fd
3. US asks GPU driver to import a buffer and provides it with dma_buf_fd
4. GPU renders contents into display buffer (dma_buf_fd)
After speaking with Oleksandr on IRC, I think the main usage of the
gntdev extension is to:

1. Create a dma-buf from a set of grant references.
2. Share dma-buf and get a list of grant references.

I think this set of operations could be broken into:

1.1 Map grant references into user-space using the gntdev.
1.2 Create a dma-buf out of a set of user-space virtual addresses.

2.1 Map a dma-buf into user-space.
2.2 Get grefs out of the user-space addresses where the dma-buf is

So it seems like what's actually missing is a way to:

  - Create a dma-buf from a list of user-space virtual addresses.
  - Allow to map a dma-buf into user-space, so it can then be used with
    the gntdev.

I think this is generic enough that it could be implemented by a
device not tied to Xen. AFAICT the hyper_dma guys also wanted
something similar to this.
You can't just wrap random userspace memory into a dma-buf. We've just had
this discussion with kvm/qemu folks, who proposed just that, and after a
bit of discussion they'll now try to have a driver which just wraps a
memfd into a dma-buf.
So, we have to decide either we introduce a new driver
(say, under drivers/xen/xen-dma-buf) or extend the existing
gntdev/balloon to support dma-buf use-cases.

Can anybody from Xen community express their preference here?

And I hope that there is no objection to have it all in the kernel,
without going to user-space with VAs and back (device-X driver)

Yes i915 and amdgpu and a few other drivers do have facilities to wrap
userspace memory into a gpu buffer object. But we don't allow those to be
exported to other drivers, because the core mm magic needed to make this
all work is way too tricky, even within the context of just 1 driver. And
dma-buf does not have the required callbacks and semantics to make it

Finally, this is indeed some memory, but a bit more [1]
Also, (with my FreeBSD maintainer hat) how is this going to translate
to other OSes? So far the operations performed by the gntdev device
are mostly OS-agnostic because this just map/unmap memory, and in fact
they are implemented by Linux and FreeBSD.
At the moment I can only see Linux implementation and it seems
to be perfectly ok as we do not change Xen's APIs etc. and only
use the existing ones (remember, we only extend gntdev/balloon
drivers, all the changes in the Linux kernel)
As the second note I can also think that we do not extend gntdev/balloon
drivers and have re-worked xen-zcopy driver be a separate entity,
say drivers/xen/dma-buf
implement "wait" ioctl (wait for dma-buf->release): currently these are
1.2. Xen balloon driver [6] to allow allocating contiguous buffers (not
by current hyper-dmabuf, but is a must for xen-zcopy use-cases)
I think this needs clarifying. In which memory space do you need those
regions to be contiguous?
Use-case: Dom0 has a HW driver which only works with contig memory
and I want DomU to be able to directly write into that memory, thus
implementing zero copying
Do they need to be contiguous in host physical memory, or guest
physical memory?
If it's in guest memory space, isn't there any generic interface that
you can use?

If it's in host physical memory space, why do you need this buffer to
be contiguous in host physical memory space? The IOMMU should hide all
There are drivers/HW which can only work with contig memory and
if it is backed by an IOMMU then still it has to be contig in IPA
space (real device doesn't know that it is actually IPA contig, not PA)
What's IPA contig?

Thanks, Roger.
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