Re: [PATCH 07/13] drm/i915: Allow userspace to clone contexts on creation

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

On 08/03/2019 14:12, Chris Wilson wrote:
> A usecase arose out of handling context recovery in mesa, whereby they
> wish to recreate a context with fresh logical state but preserving all
> other details of the original. Currently, they create a new context and
> iterate over which bits they want to copy across, but it would much more
> convenient if they were able to just pass in a target context to clone
> during creation. This essentially extends the setparam during creation
> to pull the details from a target context instead of the user supplied
> parameters.

This one is not used by media so it will likely have to find a separate 
route upstream.

> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/i915_gem_context.c | 90 +++++++++++++++++++++++++
>   include/uapi/drm/i915_drm.h             | 14 ++++
>   2 files changed, 104 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 310892b42b68..2cfc68b66944 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -1428,8 +1428,98 @@ static int create_setparam(struct i915_user_extension __user *ext, void *data)
>   	return ctx_setparam(data, &local.setparam);
>   }
>   
> +static int clone_sseu(struct i915_gem_context *dst,
> +		      struct i915_gem_context *src)
> +{
> +	const struct intel_sseu default_sseu =
> +		intel_device_default_sseu(dst->i915);
> +	struct intel_engine_cs *engine;
> +	enum intel_engine_id id;
> +
> +	for_each_engine(engine, dst->i915, id) {
> +		struct intel_context *ce;
> +		struct intel_sseu sseu;
> +
> +		ce = intel_context_lookup(src, engine);
> +		if (!ce)
> +			continue;
> +
> +		sseu = ce->sseu;
> +		if (!memcmp(&sseu, &default_sseu, sizeof(sseu)))
> +			continue;
> +
> +		ce = intel_context_pin_lock(dst, engine);
> +		if (IS_ERR(ce))
> +			return PTR_ERR(ce);
> +
> +		ce->sseu = sseu;
> +		intel_context_pin_unlock(ce);
> +	}
> +
> +	return 0;
> +}
> +
> +static int create_clone(struct i915_user_extension __user *ext, void *data)
> +{
> +	struct drm_i915_gem_context_create_ext_clone local;
> +	struct i915_gem_context *dst = data;
> +	struct i915_gem_context *src;
> +	int err;
> +
> +	if (copy_from_user(&local, ext, sizeof(local)))
> +		return -EFAULT;
> +
> +	if (local.flags & I915_CONTEXT_CLONE_UNKNOWN)
> +		return -EINVAL;
> +
> +	if (local.rsvd)
> +		return -EINVAL;
> +
> +	if (local.clone == dst->user_handle) /* good guess! denied. */
> +		return -ENOENT;

:) Good one, but put a more obvious comment like "Cannot clone itself!".

> +
> +	rcu_read_lock();
> +	src = __i915_gem_context_lookup_rcu(dst->file_priv, local.clone);
> +	rcu_read_unlock();
> +	if (!src)
> +		return -ENOENT;
> +
> +	GEM_BUG_ON(src == dst);
> +
> +	if (local.flags & I915_CONTEXT_CLONE_FLAGS)
> +		dst->user_flags = src->user_flags;
> +
> +	if (local.flags & I915_CONTEXT_CLONE_SCHED)
> +		dst->sched = src->sched;
> +
> +	if (local.flags & I915_CONTEXT_CLONE_SSEU) {
> +		err = clone_sseu(dst, src);
> +		if (err)
> +			return err;
> +	}
> +
> +	if (local.flags & I915_CONTEXT_CLONE_TIMELINE && src->timeline) {

Do we want to error out if no timeline and cloning was requested?

> +		if (dst->timeline)
> +			i915_timeline_put(dst->timeline);
> +		dst->timeline = i915_timeline_get(src->timeline);

What prevents a different thread from changing either context in 
parallel and making reference counting go bad?

> +	}
> +
> +	if (local.flags & I915_CONTEXT_CLONE_VM && src->ppgtt) {

Also fail if impossible was requested?

> +		GEM_BUG_ON(dst->ppgtt == src->ppgtt);

Hm... what prevents this? Set_vm extension followed by clone could 
trigger it I think.

> +
> +		if (dst->ppgtt)
> +			i915_ppgtt_put(dst->ppgtt);
> +
> +		dst->ppgtt = i915_ppgtt_get(src->ppgtt);
> +		i915_ppgtt_open(dst->ppgtt);

Also some locking is needed I think to make the exchange atomic.

Could use __assign_ppgtt?

> +	}
> +
> +	return 0;
> +}
> +
>   static const i915_user_extension_fn create_extensions[] = {
>   	[I915_CONTEXT_CREATE_EXT_SETPARAM] = create_setparam,
> +	[I915_CONTEXT_CREATE_EXT_CLONE] = create_clone,
>   };
>   
>   static bool client_is_banned(struct drm_i915_file_private *file_priv)
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 007d77ff7295..50d154954d5f 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -1579,6 +1579,20 @@ struct drm_i915_gem_context_create_ext_setparam {
>   	struct drm_i915_gem_context_param setparam;
>   };
>   
> +struct drm_i915_gem_context_create_ext_clone {
> +#define I915_CONTEXT_CREATE_EXT_CLONE 1
> +	struct i915_user_extension base;
> +	__u32 clone;

id, clone_id, source_id?

> +	__u32 flags;
> +#define I915_CONTEXT_CLONE_FLAGS	(1u << 0)
> +#define I915_CONTEXT_CLONE_SCHED	(1u << 1)
> +#define I915_CONTEXT_CLONE_SSEU		(1u << 2)
> +#define I915_CONTEXT_CLONE_TIMELINE	(1u << 3)
> +#define I915_CONTEXT_CLONE_VM		(1u << 4)
> +#define I915_CONTEXT_CLONE_UNKNOWN -(I915_CONTEXT_CLONE_VM << 1)
> +	__u64 rsvd;
> +};
> +
>   struct drm_i915_gem_context_destroy {
>   	__u32 ctx_id;
>   	__u32 pad;
> 

Regards,

Tvrtko
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