Re: [Intel-gfx] [PATCH 12/27] drm/i915/gem: Disallow creating contexts with too many engines

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/05/2021 16:57, Jason Ekstrand wrote:
> There's no sense in allowing userspace to create more engines than it
> can possibly access via execbuf.
> 
> Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

FWIW, again for the record, my advice is not to land this one. It makes 
no sense and it is actively disruptive for no benefit.

Regards,

Tvrtko

> ---
>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index be42dc918ef6f..8e254d83d65f2 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1640,11 +1640,11 @@ set_engines(struct i915_gem_context *ctx,
>   		return -EINVAL;
>   	}
>   
> -	/*
> -	 * Note that I915_EXEC_RING_MASK limits execbuf to only using the
> -	 * first 64 engines defined here.
> -	 */
>   	num_engines = (args->size - sizeof(*user)) / sizeof(*user->engines);
> +	/* RING_MASK has no shift so we can use it directly here */
> +	if (num_engines > I915_EXEC_RING_MASK + 1)
> +		return -EINVAL;
> +
>   	set.engines = alloc_engines(num_engines);
>   	if (!set.engines)
>   		return -ENOMEM;
> 
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