[Intel-gfx] [PATCH] drm/i915: Allow userptr backchannel for passing around GTT mappings

[Daniel Vetter]

On Thu, Oct 01, 2015 at 12:36:05PM +0100, Chris Wilson wrote:
> Once userptr becomes part of client API, it is almost a certainty that
> eventually someone will try to create a new object from a mapping of
> another client object, e.g.
> 
> new = vaImport(vaMap(old, &size), size);
> 
> (using a hypothethical API, not meaning to pick on anyone!)
> 
> Since this is actually fairly safe to implement and to allow for a GTT
> mapping (since it is within a single process space and the memory access
> passes the standard permissions test) let us not limit the Client
> possibilities.
> 
> v2: sfelling pixes
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Gwenole Beauchesne <gwenole.beauchesne at intel.com>
> Cc: Michał Winiarski <michal.winiarski at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

This feels like a really big can of worms, since all the apis I've seen
thus far have been rather explicit that you can only import malloc'ed
memory. Also tiling and stuff like this ...
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_gem_userptr.c | 48 ++++++++++++++++++++++++++++++---
>  1 file changed, 45 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> index 1b3b451b6658..afd4c2c4cc04 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -795,6 +795,37 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
>  	.release = i915_gem_userptr_release,
>  };
>  
> +static struct drm_i915_gem_object *
> +find_object_from_vma(struct drm_device *dev,
> +		     struct drm_i915_gem_userptr *args)
> +{
> +	struct drm_i915_gem_object *obj = NULL;
> +	struct vm_area_struct *vma;
> +
> +	down_read(&current->mm->mmap_sem);
> +	vma = find_vma(current->mm, args->user_ptr);
> +	if (vma == NULL) {
> +		obj = ERR_PTR(-EFAULT);
> +		goto out;
> +	}
> +
> +	if (vma->vm_ops != dev->driver->gem_vm_ops)
> +		goto out;
> +
> +	if (vma->vm_start != args->user_ptr ||
> +	    vma->vm_end != args->user_ptr + args->user_size) {
> +		obj = ERR_PTR(-EINVAL);
> +		goto out;
> +	}
> +
> +	obj = to_intel_bo(vma->vm_private_data);
> +	drm_gem_object_reference(&obj->base);
> +
> +out:
> +	up_read(&current->mm->mmap_sem);
> +	return obj;
> +}
> +
>  /**
>   * Creates a new mm object that wraps some normal memory from the process
>   * context - user memory.
> @@ -802,8 +833,11 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
>   * We impose several restrictions upon the memory being mapped
>   * into the GPU.
>   * 1. It must be page aligned (both start/end addresses, i.e ptr and size).
> - * 2. It must be normal system memory, not a pointer into another map of IO
> - *    space (e.g. it must not be a GTT mmapping of another object).
> + * 2. It must either be:
> + *    a) normal system memory, not a pointer into another map of IO
> + *       space (e.g. it must not be part of a GTT mmapping of another object).
> + *    b) a pointer to the complete GTT mmap of another object in your
> + *       address space.
>   * 3. We only allow a bo as large as we could in theory map into the GTT,
>   *    that is we limit the size to the total size of the GTT.
>   * 4. The bo is marked as being snoopable. The backing pages are left
> @@ -853,6 +887,14 @@ i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file
>  		return -ENODEV;
>  	}
>  
> +	obj = find_object_from_vma(dev, args);
> +	if (obj) {
> +		if (!IS_ERR(obj))
> +			goto out;
> +
> +		return PTR_ERR(obj);
> +	}
> +
>  	obj = i915_gem_object_alloc(dev);
>  	if (obj == NULL)
>  		return -ENOMEM;
> @@ -874,7 +916,7 @@ i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file
>  	if (ret == 0)
>  		ret = i915_gem_userptr_init__mmu_notifier(obj, args->flags);
>  	if (ret == 0)
> -		ret = drm_gem_handle_create(file, &obj->base, &handle);
> +out:		 ret = drm_gem_handle_create(file, &obj->base, &handle);
>  
>  	/* drop reference from allocate - handle holds it now */
>  	drm_gem_object_unreference_unlocked(&obj->base);
> -- 
> 2.6.0
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch