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

Fri Apr 20 11:25:20 UTC 2018

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
>>>>> safe.
>>>>> But, the following must be considered (highlighted in xen-front's Kernel
>>>>> documentation):
>>>>>    - 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
>>>>> any
>>>>>      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
>>>>> well,
>>>>>      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
>>>> attacks?
>>> 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
>>>> that:
>>>>
>>>>    - 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
>>> things
>> 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
>>>>> 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:
>>>>>     - 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
>>>>> sure
>>>>>       the buffer is freed (think of it as it waits for dma-buf->release
>>>>> callback)
>>>> 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
>>>>> error
>>>>> (BTW, wait time is a parameter of this IOCTL), Xen will defer grant
>>>>> reference
>>>>> 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
>>      mapped.
>>
>> 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
> work.
> -Daniel
>
>>> 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
>>>>> DRM_XEN_ZCOPY_DUMB_FROM_REFS, DRM_XEN_ZCOPY_DUMB_TO_REFS and
>>>>> DRM_XEN_ZCOPY_DUMB_WAIT_FREE
>>>>> 1.2. Xen balloon driver [6] to allow allocating contiguous buffers (not
>>>>> needed
>>>>> 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?
>>> Host
>>>> 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
>>>> this.
>>> 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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