Re: [Intel-gfx] [PATCH v3 2/4] drm/i915: Reserve some kernel space per vm

[Nirmoy Das <nirmoy.das@linux.intel.com>]

Hi Andrzej,

On 10/23/2023 9:41 AM, Andrzej Hajda wrote:
> Reserve two pages in each vm for kernel space to use for things
> such as workarounds.
>
> v2: use real memory, do not decrease vm.total
>
> Suggested-by: Chris Wilson <chris.p.wilson@linux.intel.com>
> Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
> Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
> ---
>   drivers/gpu/drm/i915/gt/gen8_ppgtt.c | 38 ++++++++++++++++++++++++++++++++++++
>   drivers/gpu/drm/i915/gt/intel_gtt.h  |  1 +
>   2 files changed, 39 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
> index 84aa29715e0aca..c25e1d4cceeb17 100644
> --- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
> +++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
> @@ -5,6 +5,7 @@
>   
>   #include <linux/log2.h>
>   
> +#include "gem/i915_gem_internal.h"
>   #include "gem/i915_gem_lmem.h"
>   
>   #include "gen8_ppgtt.h"
> @@ -953,6 +954,39 @@ gen8_alloc_top_pd(struct i915_address_space *vm)
>   	return ERR_PTR(err);
>   }
>   
> +static int gen8_init_rsvd(struct i915_address_space *vm)
> +{
> +	const resource_size_t size = 2 * PAGE_SIZE;
> +	struct drm_i915_private *i915 = vm->i915;
> +	struct drm_i915_gem_object *obj;
> +	struct i915_vma *vma;
> +	int ret;
> +
> +	obj = i915_gem_object_create_lmem(i915, size,
> +					  I915_BO_ALLOC_VOLATILE |
> +					  I915_BO_ALLOC_GPU_ONLY);

Please add a comment why GPU_ONLY flag is used. It makes sense to me now 
but good to have a comment for the future. Also why 2 pages are

reserved ?


Regards,

Nirmoy

> +	if (IS_ERR(obj))
> +		obj = i915_gem_object_create_internal(i915, size);
> +	if (IS_ERR(obj))
> +		return PTR_ERR(obj);
> +
> +	vma = i915_vma_instance(obj, vm, NULL);
> +	if (IS_ERR(vma)) {
> +		ret = PTR_ERR(vma);
> +		goto unref;
> +	}
> +
> +	ret = i915_vma_pin(vma, 0, 0, PIN_USER | PIN_HIGH);
> +	if (ret)
> +		goto unref;
> +
> +	vm->rsvd = i915_vma_make_unshrinkable(vma);
> +
> +unref:
> +	i915_gem_object_put(obj);
> +	return ret;
> +}
> +
>   /*
>    * GEN8 legacy ppgtt programming is accomplished through a max 4 PDP registers
>    * with a net effect resembling a 2-level page table in normal x86 terms. Each
> @@ -1034,6 +1068,10 @@ struct i915_ppgtt *gen8_ppgtt_create(struct intel_gt *gt,
>   	if (intel_vgpu_active(gt->i915))
>   		gen8_ppgtt_notify_vgt(ppgtt, true);
>   
> +	err = gen8_init_rsvd(&ppgtt->vm);
> +	if (err)
> +		goto err_put;
> +
>   	return ppgtt;
>   
>   err_put:
> diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.h b/drivers/gpu/drm/i915/gt/intel_gtt.h
> index 15c71da14d1d27..4a35ef24501b5f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_gtt.h
> +++ b/drivers/gpu/drm/i915/gt/intel_gtt.h
> @@ -250,6 +250,7 @@ struct i915_address_space {
>   	struct work_struct release_work;
>   
>   	struct drm_mm mm;
> +	struct i915_vma *rsvd;
>   	struct intel_gt *gt;
>   	struct drm_i915_private *i915;
>   	struct device *dma;
>