[PATCH] drm/doc: device hot-unplug for userspace

Andrey Grodzovsky Andrey.Grodzovsky at amd.com
Wed May 27 15:23:36 UTC 2020

On 5/27/20 10:39 AM, Daniel Vetter wrote:
> On Wed, May 27, 2020 at 3:51 PM Andrey Grodzovsky
> <Andrey.Grodzovsky at amd.com> wrote:
>> On 5/27/20 2:44 AM, Pekka Paalanen wrote:
>>> On Tue, 26 May 2020 10:30:20 -0400
>>> Andrey Grodzovsky <Andrey.Grodzovsky at amd.com> wrote:
>>>> On 5/19/20 6:06 AM, Pekka Paalanen wrote:
>>>>> From: Pekka Paalanen <pekka.paalanen at collabora.com>
>>>>> Set up the expectations on how hot-unplugging a DRM device should look like to
>>>>> userspace.
>>>>> Written by Daniel Vetter's request and largely based on his comments in IRC and
>>>>> from https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Farchives%2Fdri-devel%2F2020-May%2F265484.html&data=02%7C01%7CAndrey.Grodzovsky%40amd.com%7C3c671803b2ba41b2ceac08d8024bcc9a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637261871869742519&sdata=ZnhylRubOM0%2BjoreSSYMqVDzZuUdybEsoyBVcTKgxWE%3D&reserved=0 .
>>>>> Signed-off-by: Pekka Paalanen <pekka.paalanen at collabora.com>
>>>>> Cc: Daniel Vetter <daniel at ffwll.ch>
>>>>> Cc: Andrey Grodzovsky <andrey.grodzovsky at amd.com>
>>>>> Cc: Dave Airlie <airlied at redhat.com>
>>>>> Cc: Sean Paul <sean at poorly.run>
>>>>> ---
>>>>> Disclaimer: I am a userspace developer writing for other userspace developers.
>>>>> I took some liberties in defining what should happen without knowing what is
>>>>> actually possible or what existing drivers already implement.
>>>>> ---
>>>>>     Documentation/gpu/drm-uapi.rst | 75 ++++++++++++++++++++++++++++++++++
>>>>>     1 file changed, 75 insertions(+)
>>>>> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
>>>>> index 56fec6ed1ad8..80db4abd2cbd 100644
>>>>> --- a/Documentation/gpu/drm-uapi.rst
>>>>> +++ b/Documentation/gpu/drm-uapi.rst
>>>>> @@ -1,3 +1,5 @@
>>>>> +.. Copyright 2020 DisplayLink (UK) Ltd.
>>>>> +
>>>>>     ===================
>>>>>     Userland interfaces
>>>>>     ===================
>>>>> @@ -162,6 +164,79 @@ other hand, a driver requires shared state between clients which is
>>>>>     visible to user-space and accessible beyond open-file boundaries, they
>>>>>     cannot support render nodes.
>>>>> +Device Hot-Unplug
>>>>> +=================
>>>>> +
>>>>> +.. note::
>>>>> +   The following is the plan. Implementation is not there yet
>>>>> +   (2020 May 13).
>>>>> +
>>>>> +Graphics devices (display and/or render) may be connected via USB (e.g.
>>>>> +display adapters or docking stations) or Thunderbolt (e.g. eGPU). An end
>>>>> +user is able to hot-unplug this kind of devices while they are being
>>>>> +used, and expects that the very least the machine does not crash. Any
>>>>> +damage from hot-unplugging a DRM device needs to be limited as much as
>>>>> +possible and userspace must be given the chance to handle it if it wants
>>>>> +to. Ideally, unplugging a DRM device still lets a desktop to continue
>>>>> +running, but that is going to need explicit support throughout the whole
>>>>> +graphics stack: from kernel and userspace drivers, through display
>>>>> +servers, via window system protocols, and in applications and libraries.
>>>>> +
>>>>> +Other scenarios that should lead to the same are: unrecoverable GPU
>>>>> +crash, PCI device disappearing off the bus, or forced unbind of a driver
>>>>> +from the physical device.
>>>>> +
>>>>> +In other words, from userspace perspective everything needs to keep on
>>>>> +working more or less, until userspace stops using the disappeared DRM
>>>>> +device and closes it completely. Userspace will learn of the device
>>>>> +disappearance from the device removed uevent or in some cases specific
>>>>> +ioctls returning EIO.
>>>>> +
>>>>> +This goal raises at least the following requirements for the kernel and
>>>>> +drivers:
>>>>> +
>>>>> +- The kernel must not hang, crash or oops, no matter what userspace was
>>>>> +  in the middle of doing when the device disappeared.
>>>>> +
>>>>> +- All GPU jobs that can no longer run must have their fences
>>>>> +  force-signalled to avoid inflicting hangs to userspace.
>>>>> +
>>>>> +- KMS connectors must change their status to disconnected.
>>>>> +
>>>>> +- Legacy modesets and pageflips fake success.
>>>>> +
>>>>> +- Atomic commits, both real and TEST_ONLY, fake success.
>>>>> +
>>>>> +- Pending non-blocking KMS operations deliver the DRM events userspace
>>>>> +  is expecting.
>>>>> +
>>>>> +- If underlying memory disappears, the mmaps are replaced with harmless
>>>>> +  zero pages where access does not raise SIGBUS. Reads return zeros,
>>>>> +  writes are ignored.
>>>> Regarding this paragraph - what about exiting mappings ? In the first
>>>> patchset we would actively invalidate all the existing CPU mappings to
>>>> device memory and i think we still should do it otherwise we will see
>>>> random crashes in applications as was before. I guess it's because TLBs
>>>> and page tables are not updated to reflect the fact the device is gone.
>>> Hi,
>>> I was talking about existing mappings. What I forgot to specify is how
>>> new mmap() calls after the device disappearance should work (the end
>>> result should be the same still, not failure).
>>> I'll clarify this in the next revision.
>>> Thanks,
>>> pq
>> I see, that ok.
>> Next related question is more for Daniel/Christian - about the
>> implementation of this paragraph, I was thinking about something like
>> checking for device disconnect in ttm_bo_vm_fault_reserved and if so
>> remap the entire VA range for the VMA where the fault address belongs to
>> the global zero page (i.e. (remap_pfn_range(vma, vma->vm_start,
>> page_to_pfn(ZERO_PAGE(vma->vm_start), vma->vm_end - vma->vm_start,
>> vma->vm_page_prot)). Question is, when the doc says 'writes are ignored'
>> does it mean i should use copy on write for the vma->vm_page_prot and if
>> so how i actually do it as i was not able to find what flags to set into
>> vm_page_prot to force copy on write behavior.
> Already discussed this with Pekka on irc, I think simply a private
> page (per gpu ctx to avoid leaks) is good enough. Otherwise we need to
> catch write faults and throw the writes away, and that's a) a bit
> tricky to implement and b) slow, which we kinda don't want to. If the
> desktop is stuck for a few seconds because we're trapping every write
> of a 4k buffer that's getting uploaded, the user is going to have a
> bad time :-/
> -Daniel

So like allocating a page per process context in the driver (struct 
amdgpu_ctx in amdgpu) and mapping this page into the faulting VMAs  for 
when device is disconnected ? I am still not clear how i make the 
mapping ignore writes without catching write faults and ignoring them. I 
cannot just make it read only obviously and i can't make it writable as 
then reading back will start returning non 0's. My question is what set 
of flags in vm_area_struct.vm_flags can (if at all) give me 'ignore 
writes' behavior for the mapping of that page.


>> Andrey
>>>>> +
>>>>> +- dmabuf which point to memory that has disappeared are rewritten to
>>>>> +  point to harmless zero pages, similar to mmaps. Imports still succeed
>>>>> +  both ways: an existing device importing a dmabuf pointing to
>>>>> +  disappeared memory, and a disappeared device importing any dmabuf.
>>>>> +
>>>>> +- Render ioctls return EIO which is then handled in userspace drivers,
>>>>> +  e.g. Mesa, to have the device disappearance handled in the way
>>>>> +  specified for each API (OpenGL, GL ES: GL_KHR_robustness;
>>>>> +  Vulkan: VK_ERROR_DEVICE_LOST; etc.)
>>>>> +
>>>>> +Raising SIGBUS is not an option, because userspace cannot realistically
>>>>> +handle it.  Signal handlers are global, which makes them extremely
>>>>> +difficult to use correctly from libraries like Mesa produces. Signal
>>>>> +handlers are not composable, you can't have different handlers for GPU1
>>>>> +and GPU2 from different vendors, and a third handler for mmapped regular
>>>>> +files.  Threads cause additional pain with signal handling as well.
>>>>> +
>>>>> +Only after userspace has closed all relevant DRM device and dmabuf file
>>>>> +descriptors and removed all mmaps, the DRM driver can tear down its
>>>>> +instance for the device that no longer exists. If the same physical
>>>>> +device somehow comes back in the mean time, it shall be a new DRM
>>>>> +device.
>>>>> +
>>>>>     .. _drm_driver_ioctl:
>>>>>     IOCTL Support on Device Nodes

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