Re: [PATCH v4 2/2] gpu: nova-core: gsp: fix undefined behavior in command queue code

["Danilo Krummrich" <dakr@kernel.org>]

On Sat Apr 4, 2026 at 1:47 AM CEST, John Hubbard wrote:
> On 4/1/26 7:29 AM, Alexandre Courbot wrote:
>> `driver_read_area` and `driver_write_area` are internal methods that
>> return slices containing the area of the command queue buffer that the
>> driver has exclusive read or write access, respectively.
>> 
>> While their returned value is correct and safe to use, internally they
>> temporarily create a reference to the whole command-buffer slice,
>> including GSP-owned regions. These regions can change without notice,
>> and thus creating a slice to them, even if never accessed, is undefined
>> behavior.
>> 
>> Fix this by rewriting these methods to use pointer projections in order
>> to create slices to valid regions only. It should eventually be replaced
>> by `IoView` and `IoSlice` once they land.
>> 
>> Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
>> Reported-by: Danilo Krummrich <dakr@kernel.org>
>> Closes: https://lore.kernel.org/all/DH47AVPEKN06.3BERUSJIB4M1R@kernel.org/
>> Reviewed-by: Gary Guo <gary@garyguo.net>
>> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>> ---
>>  drivers/gpu/nova-core/gsp/cmdq.rs | 114 ++++++++++++++++++++++----------------
>>  1 file changed, 65 insertions(+), 49 deletions(-)
>
> This is causing a build_assert failure in the latest drm-rust-next, with
> rustc 1.85.0, and also with 1.78.0.
>
> rustc 1.93.0 does not show the problem.

Odd, it did pass all the testing at my end. Anyways, it only proves once again
that this is pretty fragile.

> I suggest that we revert this commit: we're very late in the cycle and
> it appears to be relying on fragile compiler behavior (my best guess so
> far--I don't yet understand the root cause).

As an exception, as it made more sense in this case, I dropped it from the
queue.

Thanks,
Danilo