[PATCH v5 20/27] drm: Scope all DRM IOCTLs with drm_dev_enter/exit

[Felix Kuehling]

Am 2021-04-29 um 12:04 p.m. schrieb Andrey Grodzovsky:
> So as I understand your preferred approach is that I scope any
> back_end, HW specific function with drm_dev_enter/exit because that
> where MMIO
> access takes place. But besides explicit MMIO access thorough
> register accessors in the HW back-end there is also indirect MMIO access
> taking place throughout the code in the driver because of various VRAM
> BOs which provide CPU access to VRAM through the VRAM BAR. This kind of
> access is spread all over in the driver and even in mid-layers such as
> TTM and not limited to HW back-end functions. It means it's much harder
> to spot such places to surgically scope them with drm_dev_enter/exit and
> also that any new such code introduced will immediately break hot unplug
> because the developers can't be expected to remember making their code
> robust to this specific use case. That why when we discussed internally
> what approach to take to protecting code with drm_dev_enter/exit we
> opted for using the widest available scope.

VRAM can also be mapped in user mode. Is there anything preventing user
mode from accessing the memory after unplug? I guess the best you could
do is unmap it from the CPU page table and let the application segfault
on the next access. Or replace the mapping with a dummy page in system
memory?

Regards,
  Felix


>
> Andrey