[RFC 0/7] drm/virtio: Import scanout buffers from other devices
Kasireddy, Vivek
vivek.kasireddy at intel.com
Thu May 30 07:21:01 UTC 2024
Hi Gurchetan,
>
> On Fri, May 24, 2024 at 11:33 AM Kasireddy, Vivek
> <vivek.kasireddy at intel.com <mailto:vivek.kasireddy at intel.com> > wrote:
>
>
> Hi,
>
> Sorry, my previous reply got messed up as a result of HTML
> formatting. This is
> a plain text version of the same reply.
>
> >
> >
> > Having virtio-gpu import scanout buffers (via prime) from other
> > devices means that we'd be adding a head to headless GPUs
> assigned
> > to a Guest VM or additional heads to regular GPU devices that
> are
> > passthrough'd to the Guest. In these cases, the Guest
> compositor
> > can render into the scanout buffer using a primary GPU and has
> the
> > secondary GPU (virtio-gpu) import it for display purposes.
> >
> > The main advantage with this is that the imported scanout
> buffer can
> > either be displayed locally on the Host (e.g, using Qemu + GTK
> UI)
> > or encoded and streamed to a remote client (e.g, Qemu + Spice
> UI).
> > Note that since Qemu uses udmabuf driver, there would be no
> > copies
> > made of the scanout buffer as it is displayed. This should be
> > possible even when it might reside in device memory such has
> > VRAM.
> >
> > The specific use-case that can be supported with this series is
> when
> > running Weston or other guest compositors with "additional-
> devices"
> > feature (./weston --drm-device=card1 --additional-
> devices=card0).
> > More info about this feature can be found at:
> > https://gitlab.freedesktop.org/wayland/weston/-
> > /merge_requests/736
> >
> > In the above scenario, card1 could be a dGPU or an iGPU and
> card0
> > would be virtio-gpu in KMS only mode. However, the case
> where this
> > patch series could be particularly useful is when card1 is a GPU
> VF
> > that needs to share its scanout buffer (in a zero-copy way) with
> the
> > GPU PF on the Host. Or, it can also be useful when the scanout
> buffer
> > needs to be shared between any two GPU devices (assuming
> one of
> > them
> > is assigned to a Guest VM) as long as they are P2P DMA
> compatible.
> >
> >
> >
> > Is passthrough iGPU-only or passthrough dGPU-only something you
> intend to
> > use?
> Our main use-case involves passthrough’g a headless dGPU VF device
> and sharing
> the Guest compositor’s scanout buffer with dGPU PF device on the
> Host. Same goal for
> headless iGPU VF to iGPU PF device as well.
>
>
>
> Just to check my understanding: the same physical {i, d}GPU is partitioned
> into the VF and PF, but the PF handles host-side display integration and
> rendering?
Yes, that is mostly right. In a nutshell, the same physical GPU is partitioned
into one PF device and multiple VF devices. Only the PF device has access to
the display hardware and can do KMS (on the Host). The VF devices are
headless with no access to display hardware (cannot do KMS but can do render/
encode/decode) and are generally assigned (or passthrough'd) to the Guest VMs.
Some more details about this model can be found here:
https://lore.kernel.org/dri-devel/20231110182231.1730-1-michal.wajdeczko@intel.com/
>
>
> However, using a combination of iGPU and dGPU where either of
> them can be passthrough’d
> to the Guest is something I think can be supported with this patch
> series as well.
>
> >
> > If it's a dGPU + iGPU setup, then the way other people seem to do it
> is a
> > "virtualized" iGPU (via virgl/gfxstream/take your pick) and pass-
> through the
> > dGPU.
> >
> > For example, AMD seems to use virgl to allocate and import into
> the dGPU.
> >
> > https://gitlab.freedesktop.org/mesa/mesa/-
> /merge_requests/23896
> >
> > https://lore.kernel.org/all/20231221100016.4022353-1-
> > julia.zhang at amd.com/ <http://julia.zhang@amd.com/>
> >
> >
> > ChromeOS also uses that method (see crrev.com/c/3764931
> <http://crrev.com/c/3764931>
> > <http://crrev.com/c/3764931> ) [cc: dGPU architect +Dominik Behr
> > <mailto:dbehr at google.com <mailto:dbehr at google.com> > ]
> >
> > So if iGPU + dGPU is the primary use case, you should be able to
> use these
> > methods as well. The model would "virtualized iGPU" +
> passthrough dGPU,
> > not split SoCs.
> In our use-case, the goal is to have only one primary GPU
> (passthrough’d iGPU/dGPU)
> do all the rendering (using native DRI drivers) for clients/compositor
> and all the outputs
> and share the scanout buffers with the secondary GPU (virtio-gpu).
> Since this is mostly
> how Mutter (and also Weston) work in a multi-GPU setup, I am not
> sure if virgl is needed.
>
>
>
> I think you can probably use virgl with the PF and others probably will, but
> supporting multiple methods in Linux is not unheard of.
In our case, we have an alternative SR-IOV based GPU virtualization/partitioning
model (as described above) where a Guest VM will have access to a hardware-accelerated
GPU VF device for its rendering/encode/decode needs. So, in this situation, using
virgl will become redundant and unnecessary.
And, in this model, we intend to use virtio-gpu for KMS in the Guest VM (since the
GPU VF device cannot do KMS) with the addition of this patchset. However, note that,
since not all GPU SKUs/versions have the SRIOV capability, we plan on using virgl in
those cases where it becomes necessary.
>
> Does your patchset need the Mesa kmsro patchset to function correctly?
>
> https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9592
This patchset is an alternative proposal. So, KMSRO would not be needed.
AFAICS, the above MR is mainly stalled because KMSRO uses dumb buffers
which are not suitable for hardware-based rendering in all cases. And, KMSRO
is not really helpful performance-wise with dGPUs, as it forces most buffers to
be allocated from system memory.
>
>
> If so, I would try to get that reviewed first to meet DRM requirements
> (https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#open-source-
> userspace-requirements). You might explicitly call out the design decision
> you're making: ("We can probably use virgl as the virtualized iGPU via PF, but
> that adds unnecessary complexity b/c ______").
As I described above, what we have is an alternative GPU virtualization scheme
where virgl is not necessary if SRIOV capability is available. And, as mentioned
earlier, I have tested this series with Mutter/Gnome-shell (upstream master)
(plus one small patch: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3745)
and no other changes to any other userspace components on Host and Guest.
>
>
> And, doing it this way means that no other userspace components
> need to be modified
> on both the Guest and the Host.
>
> >
> >
> >
> > As part of the import, the virtio-gpu driver shares the dma
> > addresses and lengths with Qemu which then determines
> whether
> > the
> > memory region they belong to is owned by a PCI device or
> whether it
> > is part of the Guest's system ram. If it is the former, it identifies
> > the devid (or bdf) and bar and provides this info (along with
> offsets
> > and sizes) to the udmabuf driver. In the latter case, instead of
> the
> > the devid and bar it provides the memfd. The udmabuf driver
> then
> > creates a dmabuf using this info that Qemu shares with Spice
> for
> > encode via Gstreamer.
> >
> > Note that the virtio-gpu driver registers a move_notify() callback
> > to track location changes associated with the scanout buffer and
> > sends attach/detach backing cmds to Qemu when appropriate.
> And,
> > synchronization (that is, ensuring that Guest and Host are not
> > using the scanout buffer at the same time) is ensured by
> pinning/
> > unpinning the dmabuf as part of plane update and using a fence
> > in resource_flush cmd.
> >
> >
> > I'm not sure how QEMU's display paths work, but with crosvm if
> you share
> > the guest-created dmabuf with the display, and the guest moves
> the backing
> > pages, the only recourse is the destroy the surface and show a
> black screen
> > to the user: not the best thing experience wise.
> Since Qemu GTK UI uses EGL, there is a blit done from the guest’s
> scanout buffer onto an EGL
> backed buffer on the Host. So, this problem would not happen as of
> now.
>
>
>
> The guest kernel doesn't know you're using the QEMU GTK UI + EGL host-
> side.
So, with blob=true, there is a dma fence in resource_flush() that gets associated
with the Blit/Encode on the Host. This guest dma fence should eventually be signalled
only when the Host is done using guest's scanout buffer.
>
> If somebody wants to use the virtio-gpu import mechanism with lower-level
> Wayland-based display integration, then the problem would occur.
Right, one way to address this issue is to prevent the Guest compositor from
reusing the scanout buffer (until the Host is done) and forcing it to pick a new
buffer (since Mesa GBM allows 4 backbuffers).
I have tried this experiment with KMSRO and Wayland-based Qemu UI previously
on iGPUs (and Weston) and noticed that the Guest FPS was getting halved:
https://lore.kernel.org/qemu-devel/20210913222036.3193732-1-vivek.kasireddy@intel.com/
and also discussed and proposed a solution which did not go anywhere:
https://lore.kernel.org/dri-devel/20210913233529.3194401-1-vivek.kasireddy@intel.com/
>
> Perhaps, do that just to be safe unless you have performance concerns.
If you meant pinning the imported scanout buffer in the Guest, then yes,
that is something I am already doing in this patchset.
>
>
> >
> > Only amdgpu calls dma_buf_move_notfiy(..), and you're probably
> testing on
> > Intel only, so you may not be hitting that code path anyways.
> I have tested with the Xe driver in the Guest which also calls
> dma_buf_move_notfiy(). But
> note that for dGPUs, both Xe and amdgpu migrate the scanout buffer
> from vram to system
> memory as part of export, because virtio-gpu is not P2P compatible.
> However, I am hoping
> to relax this (p2p check against virtio-gpu) in Xe driver if it detects
> that it is running in
> VF mode once the following patch series is merged:
> https://lore.kernel.org/dri-devel/20240422063602.3690124-1-
> vivek.kasireddy at intel.com/
>
> > I forgot the
> > exact reason, but apparently udmabuf may not work with amdgpu
> displays
> > and it seems the virtualized iGPU + dGPU is the way to go for
> amdgpu
> > anyways.
> I would really like to know why udmabuf would not work with
> amdgpu?
>
>
>
> It's just a rumor I heard, but the idea is udmabuf would be imported into
> AMDGPU_GEM_DOMAIN_CPU only.
>
> https://cgit.freedesktop.org/drm/drm-
> misc/tree/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c#n333
>
> "AMDGPU_GEM_DOMAIN_CPU: System memory that is not GPU accessible.
> Memory in this pool could be swapped out to disk if there is pressure."
>
> https://dri.freedesktop.org/docs/drm/gpu/amdgpu.html
>
>
> Perhaps that limitation is artificial and unnecessary, and it may indeed work.
> I don't think anybody has tried...
Since udmabuf driver properly pins the backing pages (from memfd) for DMA,
I don't see any reason why amdgpu would not be able to import.
Thanks,
Vivek
>
>
>
>
> > So I recommend just pinning the buffer for the lifetime of the
> > import for simplicity and correctness.
> Yeah, in this patch series, the dmabuf is indeed pinned, but only for a
> short duration in the Guest –
> just until the Host is done using it (blit or encode).
>
> Thanks,
> Vivek
>
> >
> >
> > This series is available at:
> > https://gitlab.freedesktop.org/Vivek/drm-tip/-
> > /commits/virtgpu_import_rfc
> >
> > along with additional patches for Qemu and Spice here:
> > https://gitlab.freedesktop.org/Vivek/qemu/-
> > /commits/virtgpu_dmabuf_pcidev
> > https://gitlab.freedesktop.org/Vivek/spice/-
> > /commits/encode_dmabuf_v4
> >
> > Patchset overview:
> >
> > Patch 1: Implement
> > VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING cmd
> > Patch 2-3: Helpers to initalize, import, free imported object
> > Patch 4-5: Import and use buffers from other devices for
> scanout
> > Patch 6-7: Have udmabuf driver create dmabuf from PCI bars
> for P2P
> > DMA
> >
> > This series is tested using the following method:
> > - Run Qemu with the following relevant options:
> > qemu-system-x86_64 -m 4096m ....
> > -device vfio-pci,host=0000:03:00.0
> > -device virtio-
> vga,max_outputs=1,blob=true,xres=1920,yres=1080
> > -spice port=3001,gl=on,disable-ticketing=on,preferred-
> > codec=gstreamer:h264
> > -object memory-backend-memfd,id=mem1,size=4096M
> > -machine memory-backend=mem1 ...
> > - Run upstream Weston with the following options in the Guest
> VM:
> > ./weston --drm-device=card1 --additional-devices=card0
> >
> > where card1 is a DG2 dGPU (passthrough'd and using xe driver
> in
> > Guest VM),
> > card0 is virtio-gpu and the Host is using a RPL iGPU.
> >
> > Cc: Gerd Hoffmann <kraxel at redhat.com
> <mailto:kraxel at redhat.com>
> > <mailto:kraxel at redhat.com <mailto:kraxel at redhat.com> > >
> > Cc: Dongwon Kim <dongwon.kim at intel.com
> <mailto:dongwon.kim at intel.com>
> > <mailto:dongwon.kim at intel.com
> <mailto:dongwon.kim at intel.com> > >
> > Cc: Daniel Vetter <daniel.vetter at ffwll.ch
> <mailto:daniel.vetter at ffwll.ch>
> > <mailto:daniel.vetter at ffwll.ch <mailto:daniel.vetter at ffwll.ch> > >
> > Cc: Christian Koenig <christian.koenig at amd.com
> <mailto:christian.koenig at amd.com>
> > <mailto:christian.koenig at amd.com
> <mailto:christian.koenig at amd.com> > >
> > Cc: Dmitry Osipenko <dmitry.osipenko at collabora.com
> <mailto:dmitry.osipenko at collabora.com>
> > <mailto:dmitry.osipenko at collabora.com
> <mailto:dmitry.osipenko at collabora.com> > >
> > Cc: Rob Clark <robdclark at chromium.org
> <mailto:robdclark at chromium.org>
> > <mailto:robdclark at chromium.org
> <mailto:robdclark at chromium.org> > >
> > Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com
> <mailto:thomas.hellstrom at linux.intel.com>
> > <mailto:thomas.hellstrom at linux.intel.com
> <mailto:thomas.hellstrom at linux.intel.com> > >
> > Cc: Oded Gabbay <ogabbay at kernel.org
> <mailto:ogabbay at kernel.org>
> > <mailto:ogabbay at kernel.org <mailto:ogabbay at kernel.org> > >
> > Cc: Michal Wajdeczko <michal.wajdeczko at intel.com
> <mailto:michal.wajdeczko at intel.com>
> > <mailto:michal.wajdeczko at intel.com
> <mailto:michal.wajdeczko at intel.com> > >
> > Cc: Michael Tretter <m.tretter at pengutronix.de
> <mailto:m.tretter at pengutronix.de>
> > <mailto:m.tretter at pengutronix.de
> <mailto:m.tretter at pengutronix.de> > >
> >
> > Vivek Kasireddy (7):
> > drm/virtio: Implement
> > VIRTIO_GPU_CMD_RESOURCE_DETACH_BACKING cmd
> > drm/virtio: Add a helper to map and note the dma addrs and
> > lengths
> > drm/virtio: Add helpers to initialize and free the imported
> object
> > drm/virtio: Import prime buffers from other devices as guest
> blobs
> > drm/virtio: Ensure that bo's backing store is valid while
> updating
> > plane
> > udmabuf/uapi: Add new ioctl to create a dmabuf from PCI bar
> > regions
> > udmabuf: Implement UDMABUF_CREATE_LIST_FOR_PCIDEV
> ioctl
> >
> > drivers/dma-buf/udmabuf.c | 122 ++++++++++++++++--
> > drivers/gpu/drm/virtio/virtgpu_drv.h | 8 ++
> > drivers/gpu/drm/virtio/virtgpu_plane.c | 56 ++++++++-
> > drivers/gpu/drm/virtio/virtgpu_prime.c | 167
> > ++++++++++++++++++++++++-
> > drivers/gpu/drm/virtio/virtgpu_vq.c | 15 +++
> > include/uapi/linux/udmabuf.h | 11 +-
> > 6 files changed, 368 insertions(+), 11 deletions(-)
> >
> > --
> > 2.43.0
> >
> >
>
>
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