On Wed, Apr 18, 2018 at 11:10:58AM +0100, Roger Pau MonnÃ wrote:So, we have to decide either we introduce a new driver
On Wed, Apr 18, 2018 at 11:01:12AM +0300, Oleksandr Andrushchenko wrote:You can't just wrap random userspace memory into a dma-buf. We've just had
On 04/18/2018 10:35 AM, Roger Pau MonnÃ wrote:IMO we should design protocols that do not allow such attacks instead
On Wed, Apr 18, 2018 at 09:38:39AM +0300, Oleksandr Andrushchenko wrote:I mean that we trust the backend that it can prevent Dom0
On 04/17/2018 11:57 PM, Dongwon Kim wrote:I cannot parse the above sentence:
On Tue, Apr 17, 2018 at 09:59:28AM +0200, Daniel Vetter wrote:3.2 Backend exports dma-buf to xen-front
On Mon, Apr 16, 2018 at 12:29:05PM -0700, Dongwon Kim wrote:
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
Â- If guest domain dies then pages/grants received from the backend cannot
ÂÂ be claimed back - think of it as memory lost to Dom0 (won't be used for
ÂÂ other guest)
Â- Misbehaving guest may send too many requests to the backend exhausting
ÂÂ its grant references and memory (consider this from security POV). As the
ÂÂ backend runs in the trusted domain we also assume that it is trusted as
ÂÂ e.g. must take measures to prevent DDoS attacks.
"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
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 frontendThis is the first use-case above. But there are real-world
sharing pages to the backend, and I think there's a reason why this
model is used, and it should continue to be used.
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 meansYou can live without this code at all, but this is then up to
- We need more code in the backend, which increases complexity and
chances of bugs.
- Such code/logic could be wrong, thus allowing DoS.
backend which may make Dom0 down because of DomU's frontend doing evil
of having to defend against them.
After speaking with Oleksandr on IRC, I think the main usage of theBecause there is no dma-buf UAPI which allows to track the buffer life cycle4. xen-front/backend/xen-zcopy synchronizationSo this zcopy thing keeps some kind of track of the memory usage? Why
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
XENDISPL_OP_DBUF_DESTROY command (opposite to XENDISPL_OP_DBUF_CREATE).
This call is synchronous, so xen-front expects that backend does free the
buffer pages on return.
4.2. Backend, on XENDISPL_OP_DBUF_DESTROY:
Â - closes all dumb handles/fd's of the buffer according to 
Â - issues DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE IOCTL to xen-zcopy to make
ÂÂÂ the buffer is freed (think of it as it waits for dma-buf->release
can't the user-space backend keep track of the buffer usage?
(e.g. wait until dma-buf's .release callback is called)
A dma-buf is seen by user-space as a file descriptor and you canÂ - replies to xen-front that the buffer can be destroyed.I don't know much about the dma-buf implementation in Linux, but
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 ,  to allow creating dma-buf from grefs and
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?
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
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)
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
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.
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.
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 What's IPA contig?
Also, (with my FreeBSD maintainer hat) how is this going to translateAt the moment I can only see Linux implementation and it seems
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.
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,
Use-case: Dom0 has a HW driver which only works with contig memoryimplement "wait" ioctl (wait for dma-buf->release): currently these areI think this needs clarifying. In which memory space do you need those
DRM_XEN_ZCOPY_DUMB_FROM_REFS, DRM_XEN_ZCOPY_DUMB_TO_REFS and
1.2. Xen balloon driver  to allow allocating contiguous buffers (not
by current hyper-dmabuf, but is a must for xen-zcopy use-cases)
regions to be contiguous?
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 guestHost
If it's in guest memory space, isn't there any generic interface thatThere are drivers/HW which can only work with contig memory and
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
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)
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