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Re: [Xen-devel] [PATCH 0/9] drm/xen-front: Add support for Xen PV display frontend
- To: xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx, airlied@xxxxxxxx, daniel.vetter@xxxxxxxxx, seanpaul@xxxxxxxxxxxx, gustavo@xxxxxxxxxxx, jgross@xxxxxxxx, boris.ostrovsky@xxxxxxxxxx, konrad.wilk@xxxxxxxxxx
- From: Oleksandr Andrushchenko <andr2000@xxxxxxxxx>
- Date: Mon, 26 Feb 2018 10:21:09 +0200
- Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
- Delivery-date: Mon, 26 Feb 2018 08:21:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
**
*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>
Hello!
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
use-cases.
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
environment.
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"
[10]
[1] https://wiki.xen.org/wiki/Paravirtualization_(PV)#PV_IO_Drivers
[2]
https://elixir.bootlin.com/linux/v4.16-rc2/source/include/xen/interface/io/displif.h
[3] https://github.com/xen-troops/displ_be
[4] https://github.com/xen-troops/libxenbe
[5]
https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/man/xl.cfg.pod.5.in;h=a699367779e2ae1212ff8f638eff0206ec1a1cc9;hb=refs/heads/master#l1257
[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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