[PATCH 0/6] RFC Support hot device unplug in amdgpu

Daniel Vetter daniel at ffwll.ch
Mon May 11 09:54:33 UTC 2020

On Sat, May 09, 2020 at 02:51:44PM -0400, Andrey Grodzovsky wrote:
> This RFC is a more of a proof of concept then a fully working solution as there are a few unresolved issues we are hopping to get advise on from people on the mailing list.
> Until now extracting a card either by physical extraction (e.g. eGPU with thunderbold connection or by emulation through syfs -> /sys/bus/pci/devices/device_id/remove) 
> would cause random crashes in user apps. The random crashes in apps were mostly due to the app having mapped a device backed BO into it's adress space was still 
> trying to access the BO while the backing device was gone. 
> To answer this first problem Christian suggested to fix the handling of mapped memory in the clients when the device goes away by forcibly unmap all buffers 
> the user processes has by clearing their respective VMAs mapping the device BOs. Then when the VMAs try to fill in the page tables again we check in the fault handler 
> if the device is removed and if so, return an error. This will generate a SIGBUS to the application which can then cleanly terminate. 
> This indeed was done but this in turn created a problem of kernel OOPs were the OOPSes were due to the fact that while the app was terminating because of the SIGBUS 
> it would trigger use after free in the driver by calling to accesses device structures that were already released from the pci remove sequence. 
> This we handled by introducing a 'flush' seqence during device removal were we wait for drm file reference to drop to 0 meaning all user clients directly using this device terminated. 
> With this I was able to cleanly emulate device unplug with X and glxgears running and later emulate device plug back and restart of X and glxgears.
> But this use case is only partial and as I see it all the use cases are as follwing and the questions it raises.
> 1) Application accesses a BO by opening drm file
> 	1.1) BO is mapped into applications address space (BO is CPU visible) - this one we have a solution for by invaldating BO's CPU mapping casuing SIGBUS 
> 	     and termination and waiting for drm file refcound to drop to 0 before releasing the device
> 	1.2) BO is not mapped into applcation address space (BO is CPU invisible) - no solution yet because how we force the application to terminate in this case ?
> 2) Application accesses a BO by importing a DMA-BUF
> 	2.1)  BO is mapped into applications address space (BO is CPU visible) - solution is same as 1.1 but instead of waiting for drm file release we wait for the 
> 	      imported dma-buf's file release
> 	2.2)  BO is not mapped into applcation address space (BO is CPU invisible) - our solution is to invalidate GPUVM page tables and destroy backing storage for 
>               all exported BOs which will in turn casue VM faults in the importing device and then when the importing driver will try to re-attach the imported BO to 
> 	      update mappings we return -ENODEV in the import hook which hopeffuly will cause the user app to terminate.
> 3) Applcation opens a drm file or imports a dma-bud and holds a reference but never access any BO or does access but never more after device was unplug - how would we 
>    force this applcation to termiante before proceeding with device removal code ? Otherwise the wait in pci remove just hangs for ever.
> The attached patches adress 1.1, 2.1 and 2.2, for now only 1.1 fully tested and I am still testing the others but I will be happy for any advise on all the 
> described use cases and maybe some alternative and better (more generic) approach to this like maybe obtaining PIDs of relevant processes through some revere 
> mapping from device file and exported dma-buf files and send them SIGKILL - would this make more sense or any other method ? 
> Patches 1-3 address 1.1
> Patch 4 addresses 2.1
> Pathces 5-6 address 2.2
> Reference: https://gitlab.freedesktop.org/drm/amd/-/issues/1081

So we've been working on this problem for a few years already (but it's
still not solved), I think you could have saved yourselfs some typing.

Bunch of things:
- we can't wait for userspace in the hotunplug handlers, that might never
  happen. The correct way is to untangle the lifetime of your hw driver
  for a specific struct pci_device from the drm_device lifetime.
  Infrastructure is all there now, see drm_dev_get/put, drm_dev_unplug and
  drm_dev_enter/exit. A bunch of usb/spi drivers use this 100% correctly
  now, so there's examples. Plus kerneldoc explains stuff.

- for a big driver like amdgpu doing this split up is going to be
  horrendously complex. I know, we've done it for i915, at least
  partially. I strongly recommend that you're using devm_ for managing hw
  related resources (iomap, irq, ...) as much as possible.

  For drm_device resources (mostly structures and everything related to
  that) we've just merged the drmm_ managed resources framework. There's
  some more work to be done there for various kms objects, but you can at
  least somewhat avoid tedious handrolling for everything internal

  Don't ever use devm_kzalloc and friends, I've looked at hundreds of uses
  of this in drm, they're all wrong.

- dma-buf is hilarious (and atm unfixed), dma-fence is even worse. In
  theory they're already refcounted and all and so should work, in
  practice I think we need to refcount the underlying drm_device with
  drm_dev_get/put to avoid the worst fall-out.

- One unfortunate thing with drm_dev_unplug is that the driver core is
  very opinionated and doesn't tell you whether it's a hotunplug or a
  driver unload. In the former case trying to shut down hw just wastes
  time (and might hit driver bugs), in the latter case driver engineers
  very much expect everything to be shut down.

  Right now you can only have one or the other, so this needs a module
  option hack or similar (default to the correct hotunplug behaviour for

- SIGBUS is better than crashing the kernel, but it's not even close for
  users. They still lose everything because everything crashes because in
  my experience, in practice, no one ever handles errors. There's a few
  more things on top:

  - sighandlers are global, which means only the app can use it. You can't
    use it in e.g. mesa. They're also not composable, so if you have on
    sighandler for gpu1 and a 2nd one for gpu2 (could be different vendor)
    it's all sadness. Hence "usersapce will handle SIGBUS" wont work.

  - worse, neither vk nor gl (to my knowledge) have a concept of events
    for when the gpu died. The only stuff you have is things like
    arb_robustness which says a) everything continues as if nothing
    happened b) there's a function where you can ask whether your gl
    context and all the textures/buffers are toast.

    I think that's about the only hotunplug application model we can
    realistically expect applications to support. That means _all_ errors
    need to be silently eaten by either mesa or the kernel. On i915 the
    model (also for terminally wedged gpu hangs) is that all ioctl keep
    working, mmaps keep working, and execbuf gives you an -EIO (which mesa
    eats silently iirc for arb_robustness).

  Conclusion is that SIGBUS is imo a no-go, and the only option we have is
  that a) mmaps fully keep working, doable for shmem or b) we put some
  fake memory in there (for vram or whatever), maybe even only a single
  page for all fake memory.

- you probably want arb_robustness and similar stuff in userspace as a
  first step.

- refcounting, not waiting for userspace
- nothing can fail because userspace wont handle it

That's at least my take on this mess, and what we've been pushing for over
the past few years. For kms-only drm_driver we should have achieved that
by now (plus/minus maybe some issues for dma-buf/fences, but kms-only
dma-buf/fences are simple enough that maybe we don't go boom yet).

For big gpus with rendering I think best next step would be to type up a
reasonable Gran Plan (into Documentation/gpu/todo.rst) with all the issues
and likely solutions. And then bikeshed that, since the above is just my
take on all this.

Cheers, Daniel

> Andrey Grodzovsky (6):
>   drm/ttm: Add unampping of the entire device address space
>   drm/amdgpu: Force unmap all user VMAs on device removal.
>   drm/amdgpu: Wait for all user clients
>   drm/amdgpu: Wait for all clients importing out dma-bufs.
>   drm/ttm: Add destroy flag in TTM BO eviction interface
>   drm/amdgpu: Use TTM MMs destroy interface
>  drivers/gpu/drm/amd/amdgpu/amdgpu.h         |  3 ++
>  drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c |  2 +-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_device.c  |  7 +++-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 27 ++++++++++++-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c     | 22 ++++++++--
>  drivers/gpu/drm/amd/amdgpu/amdgpu_job.c     |  9 +++++
>  drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c     |  4 ++
>  drivers/gpu/drm/amd/amdgpu/amdgpu_object.c  | 17 +++++++-
>  drivers/gpu/drm/amd/amdgpu/amdgpu_object.h  |  1 +
>  drivers/gpu/drm/nouveau/nouveau_drm.c       |  2 +-
>  drivers/gpu/drm/qxl/qxl_object.c            |  4 +-
>  drivers/gpu/drm/radeon/radeon_object.c      |  2 +-
>  drivers/gpu/drm/ttm/ttm_bo.c                | 63 +++++++++++++++++++++--------
>  drivers/gpu/drm/vmwgfx/vmwgfx_drv.c         |  6 +--
>  include/drm/ttm/ttm_bo_api.h                |  2 +-
>  include/drm/ttm/ttm_bo_driver.h             |  2 +
>  16 files changed, 139 insertions(+), 34 deletions(-)
> -- 
> 2.7.4

Daniel Vetter
Software Engineer, Intel Corporation

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