linux-kernel - Re: [PATCH 3/3] drm/amdgpu: wire up the can

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Date: Tue, 6 Feb 2024 15:29:02 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Hamza Mahfooz <hamza.mahfooz@....com>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	"Pan, Xinhui" <Xinhui.Pan@....com>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Mario Limonciello <mario.limonciello@....com>,
	Lijo Lazar <lijo.lazar@....com>,
	Srinivasan Shanmugam <srinivasan.shanmugam@....com>,
	Le Ma <le.ma@....com>,
	André Almeida <andrealmeid@...lia.com>,
	James Zhu <James.Zhu@....com>,
	Aurabindo Pillai <aurabindo.pillai@....com>,
	Alex Shi <alexs@...nel.org>, Jerry Snitselaar <jsnitsel@...hat.com>,
	Wei Liu <wei.liu@...nel.org>, Robin Murphy <robin.murphy@....com>,
	amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH 3/3] drm/amdgpu: wire up the can_remove() callback

On Fri, Feb 02, 2024 at 03:40:03PM -0800, Greg Kroah-Hartman wrote:
> On Fri, Feb 02, 2024 at 05:25:56PM -0500, Hamza Mahfooz wrote:
> > Removing an amdgpu device that still has user space references allocated
> > to it causes undefined behaviour.
> 
> Then fix that please.  There should not be anything special about your
> hardware that all of the tens of thousands of other devices can't handle
> today.
> 
> What happens when I yank your device out of a system with a pci hotplug
> bus?  You can't prevent that either, so this should not be any different
> at all.
> 
> sorry, but please, just fix your driver.

fwiw Christian König from amd already rejected this too, I have no idea
why this was submitted since the very elaborate plan I developed with a
bunch of amd folks was to fix the various lifetime lolz we still have in
drm. We unfortunately export the world of internal objects to userspace as
uabi objects with dma_buf, dma_fence and everything else, but it's all
fixable and we have the plan even documented:

https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#device-hot-unplug

So yeah anything that isn't that plan of record is very much no-go for drm
drivers. Unless we change that plan of course, but that needs a
documentation patch first and a big discussion.

Aside from an absolute massive pile of kernel-internal refcounting bugs
the really big one we agreed on after a lot of discussion is that SIGBUS
on dma-buf mmaps is no-go for drm drivers, because it would break way too
much userspace in ways which are simply not fixable (since sig handlers
are shared in a process, which means the gl/vk driver cannot use it).

Otherwise it's bog standard "fix the kernel bugs" work, just a lot of it.

Cheers, Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch