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Re: [Xen-devel] [PATCH 0/9] drm/xen-front: Add support for Xen PV display frontend


*Hi, all!*


Last *Friday* some concerns on #dri-devel were raised wrt "yet

another driver" for Xen and why not virtio-gpu. Let me highlight

on why we need a new paravirtualized driver for Xen and why we

can't just use virtio. Hope this helps the communities (both Xen

and DRI) to have better understanding of this work and our motivation.

Disclaimer: some or all of the below may sound weak argument or

not 100% correct, so any help on clarifying the below is more

than welcome ;)

1. First of all, we are targeting ARM embedded use-cases and for

ARM we do not use QEMU [1]: "...Xen on ARM is not just a straight

1:1 port of x86 Xen... Xen on ARM does not need QEMU because it does

not do any emulation. It accomplishes the goal by exploiting

virtualization support in hardware as much as possible and using

paravirtualized interfaces for IO."

That being said it is still possible to run virtio-gpu and Xen+QEMU: [2]

In this case QEMU can be used for device virtualization, e.g. network,

block, console. But these already exist as Xen para-virtualized drivers

again eliminating the need for QEMU: typical ARM system runs para-virtualized

drivers for network, block, console etc.

2. virtio-gpu requires PCI/MMIO emulation

virtio-gpu (virtio-gpu-pci) require virtio-pci, but para-virtualized device

drivers do not need this.

3. No need for 3d/virgl.

There are use-cases which either do not use OpenGL at all or will use

custom virtualization solutions allowing sharing of a real GPU with guest,

e.g. vGPU approach.

4. More freedom for buffer allocation.

As of now virtio-gpu is only capable of allocating buffers via TTM, while

there are use-cases where we need to have more freedom:

for systems which do not provide IOMMU support, but having specific

requirements for display buffers, it is possible to allocate such buffers

at backend side and share those with the frontend driver.

For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting

physically contiguous memory (in PA, not IPA), this allows implementing

zero-copying use-cases.

5. Zero-copying support at backend side

Having native Xen implementation allows implementing zero-copying use-cases

on backend side with the help of supporting driver DRM driver [3] which we

hope to upstream as well (it is not yet ready in terms of code cleanup).

6. QEMU backends for virtio-gpu cannot be used as is, e.g. guest displays

could be just a part of the final user experience. Thus, a QEMU backend

must be modified to interact, for example, with Automotive Grade Linux

display manager. So, QEMU part needs modifications.

In our use-case we have a backend which supports multi-touch and guest

display(s) and running either as a weston client (which is not supported

by QEMU at the moment?) or KMS/DRM client. This allows us to enable much

more use-cases**without the need to run QEMU.


*Thank you,*

**Oleksandr Andrushchenko*


*[1] https://wiki.xen.org/wiki/Xen_ARM_with_Virtualization_Extensions_whitepaper*


[2] https://elinux.org/R-Car/Virtualization

[3] https://github.com/xen-troops/linux/blob/ces2018/drivers/gpu/drm/xen/xen_drm_zcopy_drv.c


On 02/21/2018 10:03 AM, Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>


This patch series adds support for Xen [1] para-virtualized
frontend display driver. It implements the protocol from
include/xen/interface/io/displif.h [2].
Accompanying backend [3] is implemented as a user-space application
and its helper library [4], capable of running as a Weston client
or DRM master.
Configuration of both backend and frontend is done via
Xen guest domain configuration options [5].

* Driver limitations
  1. Configuration options 1.1 (contiguous display buffers) and 2 (backend
     allocated buffers) below are not supported at the same time.

  2. Only primary plane without additional properties is supported.

  3. Only one video mode supported which resolution is configured via XenStore.

  4. All CRTCs operate at fixed frequency of 60Hz.

* Driver modes of operation in terms of display buffers used
  Depending on the requirements for the para-virtualized environment, namely
  requirements dictated by the accompanying DRM/(v)GPU drivers running in both
  host and guest environments, number of operating modes of para-virtualized
  display driver are supported:
   - display buffers can be allocated by either frontend driver or backend
   - display buffers can be allocated to be contiguous in memory or not

  Note! Frontend driver itself has no dependency on contiguous memory for
        its operation.

* 1. Buffers allocated by the frontend driver.

  The below modes of operation are configured at compile-time via
  frontend driver's kernel configuration.

  1.1. Front driver configured to use GEM CMA helpers
       This use-case is useful when used with accompanying DRM/vGPU driver in
       guest domain which was designed to only work with contiguous buffers,
       e.g. DRM driver based on GEM CMA helpers: such drivers can only import
       contiguous PRIME buffers, thus requiring frontend driver to provide
       such. In order to implement this mode of operation para-virtualized
       frontend driver can be configured to use GEM CMA helpers.

  1.2. Front driver doesn't use GEM CMA
       If accompanying drivers can cope with non-contiguous memory then, to
       lower pressure on CMA subsystem of the kernel, driver can allocate
       buffers from system memory.

  Note! If used with accompanying DRM/(v)GPU drivers this mode of operation
    may require IOMMU support on the platform, so accompanying DRM/vGPU
    hardware can still reach display buffer memory while importing PRIME
    buffers from the frontend driver.

* 2. Buffers allocated by the backend

  This mode of operation is run-time configured via guest domain configuration
  through XenStore entries.

  For systems which do not provide IOMMU support, but having specific
  requirements for display buffers it is possible to allocate such buffers
  at backend side and share those with the frontend.
  For example, if host domain is 1:1 mapped and has DRM/GPU hardware expecting
  physically contiguous memory, this allows implementing zero-copying

I would like to thank at least, but not at last the following
people/communities who helped this driver to happen ;)

1. My team at EPAM for continuous support
2. Xen community for answering tons of questions on different
modes of operation of the driver with respect to virtualized
3. Rob Clark for "GEM allocation for para-virtualized DRM driver" [6]
4. Maarten Lankhorst for "Atomic driver and old remove FB behavior" [7]
5. Ville Syrjälä for "Questions on page flips and atomic modeset" [8]

Thank you,
Oleksandr Andrushchenko

P.S. There are two dependencies for this driver limiting some of the
use-cases which are on review now:
1. "drm/simple_kms_helper: Add {enable|disable}_vblank callback support" [9]
2. "drm/simple_kms_helper: Fix NULL pointer dereference with no active CRTC" 

[1] https://wiki.xen.org/wiki/Paravirtualization_(PV)#PV_IO_Drivers
[3] https://github.com/xen-troops/displ_be
[4] https://github.com/xen-troops/libxenbe
[6] https://lists.freedesktop.org/archives/dri-devel/2017-March/136038.html
[7] https://www.spinics.net/lists/dri-devel/msg164102.html
[8] https://www.spinics.net/lists/dri-devel/msg164463.html
[9] https://patchwork.freedesktop.org/series/38073/
[10] https://patchwork.freedesktop.org/series/38139/

Oleksandr Andrushchenko (9):
   drm/xen-front: Introduce Xen para-virtualized frontend driver
   drm/xen-front: Implement Xen bus state handling
   drm/xen-front: Read driver configuration from Xen store
   drm/xen-front: Implement Xen event channel handling
   drm/xen-front: Implement handling of shared display buffers
   drm/xen-front: Introduce DRM/KMS virtual display driver
   drm/xen-front: Implement KMS/connector handling
   drm/xen-front: Implement GEM operations
   drm/xen-front: Implement communication with backend

  drivers/gpu/drm/Kconfig                     |   2 +
  drivers/gpu/drm/Makefile                    |   1 +
  drivers/gpu/drm/xen/Kconfig                 |  30 ++
  drivers/gpu/drm/xen/Makefile                |  17 +
  drivers/gpu/drm/xen/xen_drm_front.c         | 712 ++++++++++++++++++++++++++++
  drivers/gpu/drm/xen/xen_drm_front.h         | 154 ++++++
  drivers/gpu/drm/xen/xen_drm_front_cfg.c     |  84 ++++
  drivers/gpu/drm/xen/xen_drm_front_cfg.h     |  45 ++
  drivers/gpu/drm/xen/xen_drm_front_conn.c    | 125 +++++
  drivers/gpu/drm/xen/xen_drm_front_conn.h    |  35 ++
  drivers/gpu/drm/xen/xen_drm_front_drv.c     | 294 ++++++++++++
  drivers/gpu/drm/xen/xen_drm_front_drv.h     |  73 +++
  drivers/gpu/drm/xen/xen_drm_front_evtchnl.c | 399 ++++++++++++++++
  drivers/gpu/drm/xen/xen_drm_front_evtchnl.h |  89 ++++
  drivers/gpu/drm/xen/xen_drm_front_gem.c     | 360 ++++++++++++++
  drivers/gpu/drm/xen/xen_drm_front_gem.h     |  46 ++
  drivers/gpu/drm/xen/xen_drm_front_gem_cma.c |  93 ++++
  drivers/gpu/drm/xen/xen_drm_front_kms.c     | 299 ++++++++++++
  drivers/gpu/drm/xen/xen_drm_front_kms.h     |  30 ++
  drivers/gpu/drm/xen/xen_drm_front_shbuf.c   | 430 +++++++++++++++++
  drivers/gpu/drm/xen/xen_drm_front_shbuf.h   |  80 ++++
  21 files changed, 3398 insertions(+)
  create mode 100644 drivers/gpu/drm/xen/Kconfig
  create mode 100644 drivers/gpu/drm/xen/Makefile
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_cfg.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_cfg.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_conn.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_conn.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_drv.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_drv.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_evtchnl.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_evtchnl.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_gem_cma.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_kms.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_kms.h
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_shbuf.c
  create mode 100644 drivers/gpu/drm/xen/xen_drm_front_shbuf.h

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