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

Wed May 27 13:51:05 UTC 2020

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%7Ce8e13dc4c85648e5fcd408d7fbdc5f2b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637254796242596783&sdata=bA%2FAy3VGvzNqmV1kGigNROSZQEws2E98JibDxvEICNs%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.

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