Re: [PATCH 0/5] drm/vmwgfx: Support module unload and hotunplug

[Zack Rusin <zackr@vmware.com>]

On Fri, 2021-10-08 at 22:28 +0200, Thomas Hellström wrote:
> On Fri, 2021-10-08 at 13:31 -0400, Zack Rusin wrote:
> > This is a largely trivial set that makes vmwgfx support module reload
> > and PCI hot-unplug. It also makes IGT's core_hotunplug pass instead
> > of kernel oops'ing.
> > 
> > The one "ugly" change is the "Introduce a new placement for MOB page
> > tables". It seems vmwgfx has been violating a TTM assumption that
> > TTM_PL_SYSTEM buffers are never fenced for a while. Apart from a
> > kernel
> > oops on module unload it didn't seem to wreak too much havoc, but we
> > shouldn't be abusing TTM. So to solve it we're introducing a new
> > placement, which is basically system, but can deal with fenced bo's.
> > 
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> 
> Hi, Zack,
> 
> What part of TTM doesn't allow fenced system memory currently? It was
> certainly designed to allow that and vmwgfx has been relying on that
> since the introduction of MOBs IIRC. Also i915 is currently relying on
> that.

It's the shutdown. BO's allocated through the ttm system manager might
be busy during ttm_bo_put which results in them being scheduled for a
delayed deletion. The ttm system manager is disabled before the final
delayed deletion is ran in ttm_device_fini. This results in crashes
during freeing of the BO resources because they're trying to remove
themselves from a no longer existent ttm_resource_manager (e.g. in
IGT's core_hotunplug on vmwgfx)

During review of the trivial patch that was fixing it in ttm Christian
said that system domain buffers must be idle or otherwise a number of
assumptions in ttm breaks:
https://lists.freedesktop.org/archives/dri-devel/2021-September/324027.html
And later clarified that in fact system domain buffers being fenced is
illegal from a design point of view:
https://lists.freedesktop.org/archives/dri-devel/2021-September/324697.html

z