[2/2] drm/doc: add rfc section for small BAR uapi

Commit Message

Matthew Auld Feb. 18, 2022, 11:22 a.m. UTC

Add an entry for the new uapi needed for small BAR on DG2+.

Signed-off-by: Matthew Auld <[e-mail]>
Cc: Thomas Hellström <[e-mail]>
Cc: Jon Bloomfield <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Jordan Justen <[e-mail]>
Cc: Kenneth Graunke <[e-mail]>
Cc: [e-mail]
---
 Documentation/gpu/rfc/i915_small_bar.h   | 153 +++++++++++++++++++++++
 Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
 Documentation/gpu/rfc/index.rst          |   4 +
 3 files changed, 197 insertions(+)
 create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
 create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst

Comments

Thomas Hellström Feb. 22, 2022, 10:36 a.m. UTC | #1

On 2/18/22 12:22, Matthew Auld wrote:
> Add an entry for the new uapi needed for small BAR on DG2+.
>
> Signed-off-by: Matthew Auld <[e-mail]>
> Cc: Thomas Hellström <[e-mail]>
> Cc: Jon Bloomfield <[e-mail]>
> Cc: Daniel Vetter <[e-mail]>
> Cc: Jordan Justen <[e-mail]>
> Cc: Kenneth Graunke <[e-mail]>
> Cc: [e-mail]
> ---
>   Documentation/gpu/rfc/i915_small_bar.h   | 153 +++++++++++++++++++++++
>   Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
>   Documentation/gpu/rfc/index.rst          |   4 +
>   3 files changed, 197 insertions(+)
>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst
>
> diff --git a/Documentation/gpu/rfc/i915_small_bar.h b/Documentation/gpu/rfc/i915_small_bar.h
> new file mode 100644
> index 000000000000..fa65835fd608
> --- /dev/null
> +++ b/Documentation/gpu/rfc/i915_small_bar.h
> @@ -0,0 +1,153 @@
> +/**
> + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour, with added
> + * extension support using struct i915_user_extension.
> + *
> + * Note that in the future we want to have our buffer flags here,

Does this sentence need updating, with the flags member?


>   at least for
> + * the stuff that is immutable. Previously we would have two ioctls, one to
> + * create the object with gem_create, and another to apply various parameters,
> + * however this creates some ambiguity for the params which are considered
> + * immutable. Also in general we're phasing out the various SET/GET ioctls.
> + */
> +struct __drm_i915_gem_create_ext {
> +	/**
> +	 * @size: Requested size for the object.
> +	 *
> +	 * The (page-aligned) allocated size for the object will be returned.
> +	 *
> +	 * Note that for some devices we have might have further minimum
> +	 * page-size restrictions(larger than 4K), like for device local-memory.
> +	 * However in general the final size here should always reflect any
> +	 * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
> +	 * extension to place the object in device local-memory.
> +	 */
> +	__u64 size;
> +	/**
> +	 * @handle: Returned handle for the object.
> +	 *
> +	 * Object handles are nonzero.
> +	 */
> +	__u32 handle;
> +	/**
> +	 * @flags: Optional flags.
> +	 *
> +	 * Supported values:
> +	 *
> +	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the kernel that
> +	 * the object will need to be accessed via the CPU.
> +	 *
> +	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and
> +	 * only strictly required on platforms where only some of the device
> +	 * memory is directly visible or mappable through the CPU, like on DG2+.
> +	 *
> +	 * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
> +	 * ensure we can always spill the allocation to system memory, if we
> +	 * can't place the object in the mappable part of
> +	 * I915_MEMORY_CLASS_DEVICE.
> +	 *
> +	 * Note that buffers that need to be captured with EXEC_OBJECT_CAPTURE,
> +	 * will need to enable this hint, if the object can also be placed in
> +	 * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call will
> +	 * throw an error otherwise. This also means that such objects will need
> +	 * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
> +	 *
> +	 * Without this hint, the kernel will assume that non-mappable
> +	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that the
> +	 * kernel can still migrate the object to the mappable part, as a last
> +	 * resort, if userspace ever CPU faults this object, but this might be
> +	 * expensive, and so ideally should be avoided.
> +	 */
> +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
> +	__u32 flags;
> +	/**
> +	 * @extensions: The chain of extensions to apply to this object.
> +	 *
> +	 * This will be useful in the future when we need to support several
> +	 * different extensions, and we need to apply more than one when
> +	 * creating the object. See struct i915_user_extension.
> +	 *
> +	 * If we don't supply any extensions then we get the same old gem_create
> +	 * behaviour.
> +	 *
> +	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
> +	 * struct drm_i915_gem_create_ext_memory_regions.
> +	 *
> +	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
> +	 * struct drm_i915_gem_create_ext_protected_content.
> +	 */
> +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
> +#define I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
> +	__u64 extensions;
> +};
> +
> +#define DRM_I915_QUERY_VMA_INFO	5
> +
> +/**
> + * struct __drm_i915_query_vma_info
> + *
> + * Given a vm and GTT address, lookup the corresponding vma, returning its set
> + * of attributes.
> + *
> + * .. code-block:: C
> + *
> + *	struct drm_i915_query_vma_info info = {};
> + *	struct drm_i915_query_item item = {
> + *		.data_ptr = (uintptr_t)&info,
> + *		.query_id = DRM_I915_QUERY_VMA_INFO,
> + *	};
> + *	struct drm_i915_query query = {
> + *		.num_items = 1,
> + *		.items_ptr = (uintptr_t)&item,
> + *	};
> + *	int err;
> + *
> + *	// Unlike some other types of queries, there is no need to first query
> + *	// the size of the data_ptr blob here, since we already know ahead of
> + *	// time how big this needs to be.
> + *	item.length = sizeof(info);
> + *
> + *	// Next we fill in the vm_id and ppGTT address of the vma we wish
> + *	// to query, before then firing off the query.
> + *	info.vm_id = vm_id;
> + *	info.offset = gtt_address;
> + *	err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> + *	if (err || item.length < 0) ...
> + *
> + *	// If all went well we can now inspect the returned attributes.
> + *	if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
> + */
> +struct __drm_i915_query_vma_info {
> +	/**
> +	 * @vm_id: The given vm id that contains the vma. The id is the value
> +	 * returned by the DRM_I915_GEM_VM_CREATE. See struct
> +	 * drm_i915_gem_vm_control.vm_id.
> +	 */
> +	__u32 vm_id;
> +	/** @pad: MBZ. */
> +	__u32 pad;
> +	/**
> +	 * @offset: The corresponding ppGTT address of the vma which the kernel
> +	 * will use to perform the lookup.
> +	 */
> +	__u64 offset;
> +	/**
> +	 * @attributes: The returned attributes for the given vma.
> +	 *
> +	 * Possible values:
> +	 *
> +	 * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages backing the
> +	 * vma are currently CPU accessible. If this is not set then the vma is
> +	 * currently backed by I915_MEMORY_CLASS_DEVICE memory, which the CPU
> +	 * cannot directly access(this is only possible on discrete devices with
> +	 * a small BAR). Attempting to MMAP and fault such an object will
> +	 * require the kernel first synchronising any GPU work tied to the
> +	 * object, before then migrating the pages, either to the CPU accessible
> +	 * part of I915_MEMORY_CLASS_DEVICE, or I915_MEMORY_CLASS_SYSTEM, if the
> +	 * placements permit it. See I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
> +	 *
> +	 * Note that this is inherently racy.
> +	 */
> +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
> +	__u64 attributes;
> +	/** @rsvd: MBZ */
> +	__u32 rsvd[4];
> +};
> diff --git a/Documentation/gpu/rfc/i915_small_bar.rst b/Documentation/gpu/rfc/i915_small_bar.rst
> new file mode 100644
> index 000000000000..fea92d3d69ab
> --- /dev/null
> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> @@ -0,0 +1,40 @@
> +==========================
> +I915 Small BAR RFC Section
> +==========================
> +Starting from DG2 we will have resizable BAR support for device local-memory,
> +but in some cases the final BAR size might still be smaller than the total
> +local-memory size. In such cases only part of local-memory will be CPU
> +accessible, while the remainder is only accessible via the GPU.
> +
> +I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
> +----------------------------------------------
> +New gem_create_ext flag to tell the kernel that a BO will require CPU access.
> +The becomes important when placing an object in LMEM, where underneath the
> +device has a small BAR, meaning only part of it is CPU accessible. Without this
> +flag the kernel will assume that CPU access is not required, and prioritize
> +using the non-CPU visible portion of LMEM(if present on the device).
> +
> +Related to this, we now also reject any objects marked with
> +EXEC_OBJECT_CAPTURE, which are also not tagged with NEEDS_CPU_ACCESS. This only
> +impacts DG2+.
> +
> +XXX: One open here is whether we should extend the memory region query to return
> +the CPU visible size of the region. For now the IGTs just use debugfs to query
> +the size. However, if userspace sees a real need for this then extending the
> +region query would be a lot nicer.

I guess UMD folks need to comment on this. Although I think since there 
might be a number of
clients utilizing the mappable part, and a number of buffers pinned in 
there, I figure this might be of limited value outside of tests without 
some kind of cgroups support.

Otherwise

Acked-by: Thomas Hellström <[e-mail]>


> +
> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> +   :functions: __drm_i915_gem_create_ext
> +
> +DRM_I915_QUERY_VMA_INFO query
> +-----------------------------
> +Query the attributes of some vma. Given a vm and GTT offset, find the
> +respective vma, and return its set of attrubutes. For now we only support
> +DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the object/vma is
> +currently placed in memory that is accessible by the CPU. This should always be
> +set on devices where the CPU visible size of LMEM matches the probed size. If
> +this is not set then CPU faulting the object will first require migrating the
> +pages.
> +
> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> +   :functions: __drm_i915_query_vma_info
> diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
> index 018a8bf317a6..5b8495bdc1fd 100644
> --- a/Documentation/gpu/rfc/index.rst
> +++ b/Documentation/gpu/rfc/index.rst
> @@ -19,3 +19,7 @@ host such documentation:
>   .. toctree::
>   
>       i915_scheduler.rst
> +
> +.. toctree::
> +
> +    i915_small_bar.rst

Abodunrin, Akeem G Feb. 22, 2022, 5:39 p.m. UTC | #2

> -----Original Message-----
> From: dri-devel <[e-mail]> On Behalf Of
> Thomas Hellström
> Sent: Tuesday, February 22, 2022 2:36 AM
> To: Auld, Matthew <[e-mail]>; [e-mail]
> Cc: Daniel Vetter <[e-mail]>; [e-mail];
> Kenneth Graunke <[e-mail]>; Bloomfield, Jon
> <[e-mail]>; Justen, Jordan L <[e-mail]>;
> [e-mail]
> Subject: Re: [PATCH 2/2] drm/doc: add rfc section for small BAR uapi
> 
> 
> On 2/18/22 12:22, Matthew Auld wrote:
> > Add an entry for the new uapi needed for small BAR on DG2+.
> >
> > Signed-off-by: Matthew Auld <[e-mail]>
> > Cc: Thomas Hellström <[e-mail]>
> > Cc: Jon Bloomfield <[e-mail]>
> > Cc: Daniel Vetter <[e-mail]>
> > Cc: Jordan Justen <[e-mail]>
> > Cc: Kenneth Graunke <[e-mail]>
> > Cc: [e-mail]
> > ---
> >   Documentation/gpu/rfc/i915_small_bar.h   | 153
> +++++++++++++++++++++++
> >   Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
> >   Documentation/gpu/rfc/index.rst          |   4 +
> >   3 files changed, 197 insertions(+)
> >   create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
> >   create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst
> >
> > diff --git a/Documentation/gpu/rfc/i915_small_bar.h
> > b/Documentation/gpu/rfc/i915_small_bar.h
> > new file mode 100644
> > index 000000000000..fa65835fd608
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.h
> > @@ -0,0 +1,153 @@
> > +/**
> > + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour,
> > +with added
> > + * extension support using struct i915_user_extension.
> > + *
> > + * Note that in the future we want to have our buffer flags here,
> 
> Does this sentence need updating, with the flags member?
> 
> 
> >   at least for
> > + * the stuff that is immutable. Previously we would have two ioctls,
> > +one to
> > + * create the object with gem_create, and another to apply various
> > +parameters,
> > + * however this creates some ambiguity for the params which are
> > +considered
> > + * immutable. Also in general we're phasing out the various SET/GET ioctls.
> > + */
> > +struct __drm_i915_gem_create_ext {
> > +	/**
> > +	 * @size: Requested size for the object.
> > +	 *
> > +	 * The (page-aligned) allocated size for the object will be returned.
> > +	 *
> > +	 * Note that for some devices we have might have further minimum
> > +	 * page-size restrictions(larger than 4K), like for device local-memory.
> > +	 * However in general the final size here should always reflect any
> > +	 * rounding up, if for example using the
> I915_GEM_CREATE_EXT_MEMORY_REGIONS
> > +	 * extension to place the object in device local-memory.
> > +	 */
> > +	__u64 size;
> > +	/**
> > +	 * @handle: Returned handle for the object.
> > +	 *
> > +	 * Object handles are nonzero.
> > +	 */
> > +	__u32 handle;
> > +	/**
> > +	 * @flags: Optional flags.
> > +	 *
> > +	 * Supported values:
> > +	 *
> > +	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the
> kernel that
> > +	 * the object will need to be accessed via the CPU.
> > +	 *
> > +	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE,
> and
> > +	 * only strictly required on platforms where only some of the device
> > +	 * memory is directly visible or mappable through the CPU, like on DG2+.
> > +	 *
> > +	 * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM,
> to
> > +	 * ensure we can always spill the allocation to system memory, if we
> > +	 * can't place the object in the mappable part of
> > +	 * I915_MEMORY_CLASS_DEVICE.
> > +	 *
> > +	 * Note that buffers that need to be captured with
> EXEC_OBJECT_CAPTURE,
> > +	 * will need to enable this hint, if the object can also be placed in
> > +	 * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call
> will
> > +	 * throw an error otherwise. This also means that such objects will need
> > +	 * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
> > +	 *
> > +	 * Without this hint, the kernel will assume that non-mappable
> > +	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that
> the
> > +	 * kernel can still migrate the object to the mappable part, as a last
> > +	 * resort, if userspace ever CPU faults this object, but this might be
> > +	 * expensive, and so ideally should be avoided.
> > +	 */
> > +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
> > +	__u32 flags;
> > +	/**
> > +	 * @extensions: The chain of extensions to apply to this object.
> > +	 *
> > +	 * This will be useful in the future when we need to support several
> > +	 * different extensions, and we need to apply more than one when
> > +	 * creating the object. See struct i915_user_extension.
> > +	 *
> > +	 * If we don't supply any extensions then we get the same old
> gem_create
> > +	 * behaviour.
> > +	 *
> > +	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
> > +	 * struct drm_i915_gem_create_ext_memory_regions.
> > +	 *
> > +	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
> > +	 * struct drm_i915_gem_create_ext_protected_content.
> > +	 */
> > +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0 #define
> > +I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
> > +	__u64 extensions;
> > +};
> > +
> > +#define DRM_I915_QUERY_VMA_INFO	5
> > +
> > +/**
> > + * struct __drm_i915_query_vma_info
> > + *
> > + * Given a vm and GTT address, lookup the corresponding vma,
> > +returning its set
> > + * of attributes.
> > + *
> > + * .. code-block:: C
> > + *
> > + *	struct drm_i915_query_vma_info info = {};
> > + *	struct drm_i915_query_item item = {
> > + *		.data_ptr = (uintptr_t)&info,
> > + *		.query_id = DRM_I915_QUERY_VMA_INFO,
> > + *	};
> > + *	struct drm_i915_query query = {
> > + *		.num_items = 1,
> > + *		.items_ptr = (uintptr_t)&item,
> > + *	};
> > + *	int err;
> > + *
> > + *	// Unlike some other types of queries, there is no need to first query
> > + *	// the size of the data_ptr blob here, since we already know ahead of
> > + *	// time how big this needs to be.
> > + *	item.length = sizeof(info);
> > + *
> > + *	// Next we fill in the vm_id and ppGTT address of the vma we wish
> > + *	// to query, before then firing off the query.
> > + *	info.vm_id = vm_id;
> > + *	info.offset = gtt_address;
> > + *	err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> > + *	if (err || item.length < 0) ...
> > + *
> > + *	// If all went well we can now inspect the returned attributes.
> > + *	if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
> > + */
> > +struct __drm_i915_query_vma_info {
> > +	/**
> > +	 * @vm_id: The given vm id that contains the vma. The id is the value
> > +	 * returned by the DRM_I915_GEM_VM_CREATE. See struct
> > +	 * drm_i915_gem_vm_control.vm_id.
> > +	 */
> > +	__u32 vm_id;
> > +	/** @pad: MBZ. */
> > +	__u32 pad;
> > +	/**
> > +	 * @offset: The corresponding ppGTT address of the vma which the
> kernel
> > +	 * will use to perform the lookup.
> > +	 */
> > +	__u64 offset;
> > +	/**
> > +	 * @attributes: The returned attributes for the given vma.
> > +	 *
> > +	 * Possible values:
> > +	 *
> > +	 * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages
> backing the
> > +	 * vma are currently CPU accessible. If this is not set then the vma is
> > +	 * currently backed by I915_MEMORY_CLASS_DEVICE memory, which
> the CPU
> > +	 * cannot directly access(this is only possible on discrete devices with
> > +	 * a small BAR). Attempting to MMAP and fault such an object will
> > +	 * require the kernel first synchronising any GPU work tied to the
> > +	 * object, before then migrating the pages, either to the CPU accessible
> > +	 * part of I915_MEMORY_CLASS_DEVICE, or
> I915_MEMORY_CLASS_SYSTEM, if the
> > +	 * placements permit it. See
> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
> > +	 *
> > +	 * Note that this is inherently racy.
> > +	 */
> > +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
> > +	__u64 attributes;
> > +	/** @rsvd: MBZ */
> > +	__u32 rsvd[4];
> > +};
> > diff --git a/Documentation/gpu/rfc/i915_small_bar.rst
> > b/Documentation/gpu/rfc/i915_small_bar.rst
> > new file mode 100644
> > index 000000000000..fea92d3d69ab
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> > @@ -0,0 +1,40 @@
> > +==========================
> > +I915 Small BAR RFC Section
> > +==========================
> > +Starting from DG2 we will have resizable BAR support for device
> > +local-memory, but in some cases the final BAR size might still be
> > +smaller than the total local-memory size. In such cases only part of
> > +local-memory will be CPU accessible, while the remainder is only accessible
> via the GPU.
... In such cases only part of local-memory will be CPU accessible/ In such cases, only {small} part of local-memory will be CPU accessible?

> > +
> > +I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
> > +----------------------------------------------
> > +New gem_create_ext flag to tell the kernel that a BO will require CPU access.
> > +The becomes important when placing an object in LMEM, where
The/This?

> > +underneath the device has a small BAR, meaning only part of it is CPU
> > +accessible. Without this flag the kernel will assume that CPU access
> > +is not required, and prioritize using the non-CPU visible portion of LMEM(if
> present on the device).
> > +
> > +Related to this, we now also reject any objects marked with
> > +EXEC_OBJECT_CAPTURE, which are also not tagged with
> NEEDS_CPU_ACCESS.
> > +This only impacts DG2+.
> > +
> > +XXX: One open here is whether we should extend the memory region
> > +query to return the CPU visible size of the region. For now the IGTs
> > +just use debugfs to query the size. However, if userspace sees a real
> > +need for this then extending the region query would be a lot nicer.
> 
> I guess UMD folks need to comment on this. Although I think since there might
> be a number of clients utilizing the mappable part, and a number of buffers
> pinned in there, I figure this might be of limited value outside of tests without
> some kind of cgroups support.
> 
> Otherwise
> 
> Acked-by: Thomas Hellström <[e-mail]>
> 
> 
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > +   :functions: __drm_i915_gem_create_ext
> > +
> > +DRM_I915_QUERY_VMA_INFO query
> > +-----------------------------
> > +Query the attributes of some vma. Given a vm and GTT offset, find the
> > +respective vma, and return its set of attrubutes. For now we only
Attrubutes/attributes?

You might want to read the document again, and fix punctuations in addition to those typos - at least to make it more legible... 

Thanks,
~Akeem

> > +support DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the
> > +object/vma is currently placed in memory that is accessible by the
> > +CPU. This should always be set on devices where the CPU visible size
> > +of LMEM matches the probed size. If this is not set then CPU faulting
> > +the object will first require migrating the pages.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > +   :functions: __drm_i915_query_vma_info
> > diff --git a/Documentation/gpu/rfc/index.rst
> > b/Documentation/gpu/rfc/index.rst index 018a8bf317a6..5b8495bdc1fd
> > 100644
> > --- a/Documentation/gpu/rfc/index.rst
> > +++ b/Documentation/gpu/rfc/index.rst
> > @@ -19,3 +19,7 @@ host such documentation:
> >   .. toctree::
> >
> >       i915_scheduler.rst
> > +
> > +.. toctree::
> > +
> > +    i915_small_bar.rst

Lionel Landwerlin March 18, 2022, 9:38 a.m. UTC | #3

Hey Matthew, all,

This sounds like a good thing to have.
There are a number of DG2 machines where we have a small BAR and this is 
causing more apps to fail.

Anv currently reports 3 memory heaps to the app :

     - local device only (not host visible) -> mapped to lmem
     - device/cpu -> mapped to smem
     - local device but also host visible -> mapped to lmem

So we could use this straight away, by just not putting the 
I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag on the allocation of the 
first heap.

One thing I don't see in this proposal is how can we get the size of the 
2 lmem heap : cpu visible, cpu not visible
We could use that to report the appropriate size to the app.
We probably want to report a new drm_i915_memory_region_info and either :
     - put one of the reserve field to use to indicate : cpu visible
     - or define a new enum value in drm_i915_gem_memory_class

Cheers,

-Lionel


On 18/02/2022 13:22, Matthew Auld wrote:
> Add an entry for the new uapi needed for small BAR on DG2+.
>
> Signed-off-by: Matthew Auld <[e-mail]>
> Cc: Thomas Hellström <[e-mail]>
> Cc: Jon Bloomfield <[e-mail]>
> Cc: Daniel Vetter <[e-mail]>
> Cc: Jordan Justen <[e-mail]>
> Cc: Kenneth Graunke <[e-mail]>
> Cc: [e-mail]
> ---
>   Documentation/gpu/rfc/i915_small_bar.h   | 153 +++++++++++++++++++++++
>   Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
>   Documentation/gpu/rfc/index.rst          |   4 +
>   3 files changed, 197 insertions(+)
>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst
>
> diff --git a/Documentation/gpu/rfc/i915_small_bar.h b/Documentation/gpu/rfc/i915_small_bar.h
> new file mode 100644
> index 000000000000..fa65835fd608
> --- /dev/null
> +++ b/Documentation/gpu/rfc/i915_small_bar.h
> @@ -0,0 +1,153 @@
> +/**
> + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour, with added
> + * extension support using struct i915_user_extension.
> + *
> + * Note that in the future we want to have our buffer flags here, at least for
> + * the stuff that is immutable. Previously we would have two ioctls, one to
> + * create the object with gem_create, and another to apply various parameters,
> + * however this creates some ambiguity for the params which are considered
> + * immutable. Also in general we're phasing out the various SET/GET ioctls.
> + */
> +struct __drm_i915_gem_create_ext {
> +	/**
> +	 * @size: Requested size for the object.
> +	 *
> +	 * The (page-aligned) allocated size for the object will be returned.
> +	 *
> +	 * Note that for some devices we have might have further minimum
> +	 * page-size restrictions(larger than 4K), like for device local-memory.
> +	 * However in general the final size here should always reflect any
> +	 * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
> +	 * extension to place the object in device local-memory.
> +	 */
> +	__u64 size;
> +	/**
> +	 * @handle: Returned handle for the object.
> +	 *
> +	 * Object handles are nonzero.
> +	 */
> +	__u32 handle;
> +	/**
> +	 * @flags: Optional flags.
> +	 *
> +	 * Supported values:
> +	 *
> +	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the kernel that
> +	 * the object will need to be accessed via the CPU.
> +	 *
> +	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and
> +	 * only strictly required on platforms where only some of the device
> +	 * memory is directly visible or mappable through the CPU, like on DG2+.
> +	 *
> +	 * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
> +	 * ensure we can always spill the allocation to system memory, if we
> +	 * can't place the object in the mappable part of
> +	 * I915_MEMORY_CLASS_DEVICE.
> +	 *
> +	 * Note that buffers that need to be captured with EXEC_OBJECT_CAPTURE,
> +	 * will need to enable this hint, if the object can also be placed in
> +	 * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call will
> +	 * throw an error otherwise. This also means that such objects will need
> +	 * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
> +	 *
> +	 * Without this hint, the kernel will assume that non-mappable
> +	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that the
> +	 * kernel can still migrate the object to the mappable part, as a last
> +	 * resort, if userspace ever CPU faults this object, but this might be
> +	 * expensive, and so ideally should be avoided.
> +	 */
> +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
> +	__u32 flags;
> +	/**
> +	 * @extensions: The chain of extensions to apply to this object.
> +	 *
> +	 * This will be useful in the future when we need to support several
> +	 * different extensions, and we need to apply more than one when
> +	 * creating the object. See struct i915_user_extension.
> +	 *
> +	 * If we don't supply any extensions then we get the same old gem_create
> +	 * behaviour.
> +	 *
> +	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
> +	 * struct drm_i915_gem_create_ext_memory_regions.
> +	 *
> +	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
> +	 * struct drm_i915_gem_create_ext_protected_content.
> +	 */
> +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
> +#define I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
> +	__u64 extensions;
> +};
> +
> +#define DRM_I915_QUERY_VMA_INFO	5
> +
> +/**
> + * struct __drm_i915_query_vma_info
> + *
> + * Given a vm and GTT address, lookup the corresponding vma, returning its set
> + * of attributes.
> + *
> + * .. code-block:: C
> + *
> + *	struct drm_i915_query_vma_info info = {};
> + *	struct drm_i915_query_item item = {
> + *		.data_ptr = (uintptr_t)&info,
> + *		.query_id = DRM_I915_QUERY_VMA_INFO,
> + *	};
> + *	struct drm_i915_query query = {
> + *		.num_items = 1,
> + *		.items_ptr = (uintptr_t)&item,
> + *	};
> + *	int err;
> + *
> + *	// Unlike some other types of queries, there is no need to first query
> + *	// the size of the data_ptr blob here, since we already know ahead of
> + *	// time how big this needs to be.
> + *	item.length = sizeof(info);
> + *
> + *	// Next we fill in the vm_id and ppGTT address of the vma we wish
> + *	// to query, before then firing off the query.
> + *	info.vm_id = vm_id;
> + *	info.offset = gtt_address;
> + *	err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
> + *	if (err || item.length < 0) ...
> + *
> + *	// If all went well we can now inspect the returned attributes.
> + *	if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
> + */
> +struct __drm_i915_query_vma_info {
> +	/**
> +	 * @vm_id: The given vm id that contains the vma. The id is the value
> +	 * returned by the DRM_I915_GEM_VM_CREATE. See struct
> +	 * drm_i915_gem_vm_control.vm_id.
> +	 */
> +	__u32 vm_id;
> +	/** @pad: MBZ. */
> +	__u32 pad;
> +	/**
> +	 * @offset: The corresponding ppGTT address of the vma which the kernel
> +	 * will use to perform the lookup.
> +	 */
> +	__u64 offset;
> +	/**
> +	 * @attributes: The returned attributes for the given vma.
> +	 *
> +	 * Possible values:
> +	 *
> +	 * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages backing the
> +	 * vma are currently CPU accessible. If this is not set then the vma is
> +	 * currently backed by I915_MEMORY_CLASS_DEVICE memory, which the CPU
> +	 * cannot directly access(this is only possible on discrete devices with
> +	 * a small BAR). Attempting to MMAP and fault such an object will
> +	 * require the kernel first synchronising any GPU work tied to the
> +	 * object, before then migrating the pages, either to the CPU accessible
> +	 * part of I915_MEMORY_CLASS_DEVICE, or I915_MEMORY_CLASS_SYSTEM, if the
> +	 * placements permit it. See I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
> +	 *
> +	 * Note that this is inherently racy.
> +	 */
> +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
> +	__u64 attributes;
> +	/** @rsvd: MBZ */
> +	__u32 rsvd[4];
> +};
> diff --git a/Documentation/gpu/rfc/i915_small_bar.rst b/Documentation/gpu/rfc/i915_small_bar.rst
> new file mode 100644
> index 000000000000..fea92d3d69ab
> --- /dev/null
> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> @@ -0,0 +1,40 @@
> +==========================
> +I915 Small BAR RFC Section
> +==========================
> +Starting from DG2 we will have resizable BAR support for device local-memory,
> +but in some cases the final BAR size might still be smaller than the total
> +local-memory size. In such cases only part of local-memory will be CPU
> +accessible, while the remainder is only accessible via the GPU.
> +
> +I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
> +----------------------------------------------
> +New gem_create_ext flag to tell the kernel that a BO will require CPU access.
> +The becomes important when placing an object in LMEM, where underneath the
> +device has a small BAR, meaning only part of it is CPU accessible. Without this
> +flag the kernel will assume that CPU access is not required, and prioritize
> +using the non-CPU visible portion of LMEM(if present on the device).
> +
> +Related to this, we now also reject any objects marked with
> +EXEC_OBJECT_CAPTURE, which are also not tagged with NEEDS_CPU_ACCESS. This only
> +impacts DG2+.
> +
> +XXX: One open here is whether we should extend the memory region query to return
> +the CPU visible size of the region. For now the IGTs just use debugfs to query
> +the size. However, if userspace sees a real need for this then extending the
> +region query would be a lot nicer.
> +
> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> +   :functions: __drm_i915_gem_create_ext
> +
> +DRM_I915_QUERY_VMA_INFO query
> +-----------------------------
> +Query the attributes of some vma. Given a vm and GTT offset, find the
> +respective vma, and return its set of attrubutes. For now we only support
> +DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the object/vma is
> +currently placed in memory that is accessible by the CPU. This should always be
> +set on devices where the CPU visible size of LMEM matches the probed size. If
> +this is not set then CPU faulting the object will first require migrating the
> +pages.
> +
> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> +   :functions: __drm_i915_query_vma_info
> diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
> index 018a8bf317a6..5b8495bdc1fd 100644
> --- a/Documentation/gpu/rfc/index.rst
> +++ b/Documentation/gpu/rfc/index.rst
> @@ -19,3 +19,7 @@ host such documentation:
>   .. toctree::
>   
>       i915_scheduler.rst
> +
> +.. toctree::
> +
> +    i915_small_bar.rst

Matthew Auld March 18, 2022, 10:21 a.m. UTC | #4

On 18/03/2022 09:38, Lionel Landwerlin wrote:
> Hey Matthew, all,
> 
> This sounds like a good thing to have.
> There are a number of DG2 machines where we have a small BAR and this is 
> causing more apps to fail.
> 
> Anv currently reports 3 memory heaps to the app :
> 
>      - local device only (not host visible) -> mapped to lmem
>      - device/cpu -> mapped to smem
>      - local device but also host visible -> mapped to lmem
> 
> So we could use this straight away, by just not putting the 
> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag on the allocation of the 
> first heap.
> 
> One thing I don't see in this proposal is how can we get the size of the 
> 2 lmem heap : cpu visible, cpu not visible
> We could use that to report the appropriate size to the app.
> We probably want to report a new drm_i915_memory_region_info and either :
>      - put one of the reserve field to use to indicate : cpu visible
>      - or define a new enum value in drm_i915_gem_memory_class

Thanks for taking a look at this. Returning the probed CPU visible size 
as part of the region query seems reasonable. Something like:

@@ -3074,8 +3074,18 @@ struct drm_i915_memory_region_info {
         /** @unallocated_size: Estimate of memory remaining (-1 = 
unknown) */
         __u64 unallocated_size;

-       /** @rsvd1: MBZ */
-       __u64 rsvd1[8];
+       union {
+               /** @rsvd1: MBZ */
+               __u64 rsvd1[8];
+
+               struct {
+                       /**
+                        * @probed_cpu_visible_size: Memory probed by 
the driver
+                        * that is CPU accessible. (-1 = unknown)
+                        */
+                       __u64 probed_cpu_visible_size;
+               };
+       };


I will add this in the next version, if no objections.

> 
> Cheers,
> 
> -Lionel
> 
> 
> On 18/02/2022 13:22, Matthew Auld wrote:
>> Add an entry for the new uapi needed for small BAR on DG2+.
>>
>> Signed-off-by: Matthew Auld <[e-mail]>
>> Cc: Thomas Hellström <[e-mail]>
>> Cc: Jon Bloomfield <[e-mail]>
>> Cc: Daniel Vetter <[e-mail]>
>> Cc: Jordan Justen <[e-mail]>
>> Cc: Kenneth Graunke <[e-mail]>
>> Cc: [e-mail]
>> ---
>>   Documentation/gpu/rfc/i915_small_bar.h   | 153 +++++++++++++++++++++++
>>   Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
>>   Documentation/gpu/rfc/index.rst          |   4 +
>>   3 files changed, 197 insertions(+)
>>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
>>   create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst
>>
>> diff --git a/Documentation/gpu/rfc/i915_small_bar.h 
>> b/Documentation/gpu/rfc/i915_small_bar.h
>> new file mode 100644
>> index 000000000000..fa65835fd608
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>> @@ -0,0 +1,153 @@
>> +/**
>> + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour, 
>> with added
>> + * extension support using struct i915_user_extension.
>> + *
>> + * Note that in the future we want to have our buffer flags here, at 
>> least for
>> + * the stuff that is immutable. Previously we would have two ioctls, 
>> one to
>> + * create the object with gem_create, and another to apply various 
>> parameters,
>> + * however this creates some ambiguity for the params which are 
>> considered
>> + * immutable. Also in general we're phasing out the various SET/GET 
>> ioctls.
>> + */
>> +struct __drm_i915_gem_create_ext {
>> +    /**
>> +     * @size: Requested size for the object.
>> +     *
>> +     * The (page-aligned) allocated size for the object will be 
>> returned.
>> +     *
>> +     * Note that for some devices we have might have further minimum
>> +     * page-size restrictions(larger than 4K), like for device 
>> local-memory.
>> +     * However in general the final size here should always reflect any
>> +     * rounding up, if for example using the 
>> I915_GEM_CREATE_EXT_MEMORY_REGIONS
>> +     * extension to place the object in device local-memory.
>> +     */
>> +    __u64 size;
>> +    /**
>> +     * @handle: Returned handle for the object.
>> +     *
>> +     * Object handles are nonzero.
>> +     */
>> +    __u32 handle;
>> +    /**
>> +     * @flags: Optional flags.
>> +     *
>> +     * Supported values:
>> +     *
>> +     * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the 
>> kernel that
>> +     * the object will need to be accessed via the CPU.
>> +     *
>> +     * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and
>> +     * only strictly required on platforms where only some of the device
>> +     * memory is directly visible or mappable through the CPU, like 
>> on DG2+.
>> +     *
>> +     * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
>> +     * ensure we can always spill the allocation to system memory, if we
>> +     * can't place the object in the mappable part of
>> +     * I915_MEMORY_CLASS_DEVICE.
>> +     *
>> +     * Note that buffers that need to be captured with 
>> EXEC_OBJECT_CAPTURE,
>> +     * will need to enable this hint, if the object can also be 
>> placed in
>> +     * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call 
>> will
>> +     * throw an error otherwise. This also means that such objects 
>> will need
>> +     * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
>> +     *
>> +     * Without this hint, the kernel will assume that non-mappable
>> +     * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note 
>> that the
>> +     * kernel can still migrate the object to the mappable part, as a 
>> last
>> +     * resort, if userspace ever CPU faults this object, but this 
>> might be
>> +     * expensive, and so ideally should be avoided.
>> +     */
>> +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
>> +    __u32 flags;
>> +    /**
>> +     * @extensions: The chain of extensions to apply to this object.
>> +     *
>> +     * This will be useful in the future when we need to support several
>> +     * different extensions, and we need to apply more than one when
>> +     * creating the object. See struct i915_user_extension.
>> +     *
>> +     * If we don't supply any extensions then we get the same old 
>> gem_create
>> +     * behaviour.
>> +     *
>> +     * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
>> +     * struct drm_i915_gem_create_ext_memory_regions.
>> +     *
>> +     * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
>> +     * struct drm_i915_gem_create_ext_protected_content.
>> +     */
>> +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
>> +#define I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
>> +    __u64 extensions;
>> +};
>> +
>> +#define DRM_I915_QUERY_VMA_INFO    5
>> +
>> +/**
>> + * struct __drm_i915_query_vma_info
>> + *
>> + * Given a vm and GTT address, lookup the corresponding vma, 
>> returning its set
>> + * of attributes.
>> + *
>> + * .. code-block:: C
>> + *
>> + *    struct drm_i915_query_vma_info info = {};
>> + *    struct drm_i915_query_item item = {
>> + *        .data_ptr = (uintptr_t)&info,
>> + *        .query_id = DRM_I915_QUERY_VMA_INFO,
>> + *    };
>> + *    struct drm_i915_query query = {
>> + *        .num_items = 1,
>> + *        .items_ptr = (uintptr_t)&item,
>> + *    };
>> + *    int err;
>> + *
>> + *    // Unlike some other types of queries, there is no need to 
>> first query
>> + *    // the size of the data_ptr blob here, since we already know 
>> ahead of
>> + *    // time how big this needs to be.
>> + *    item.length = sizeof(info);
>> + *
>> + *    // Next we fill in the vm_id and ppGTT address of the vma we wish
>> + *    // to query, before then firing off the query.
>> + *    info.vm_id = vm_id;
>> + *    info.offset = gtt_address;
>> + *    err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
>> + *    if (err || item.length < 0) ...
>> + *
>> + *    // If all went well we can now inspect the returned attributes.
>> + *    if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
>> + */
>> +struct __drm_i915_query_vma_info {
>> +    /**
>> +     * @vm_id: The given vm id that contains the vma. The id is the 
>> value
>> +     * returned by the DRM_I915_GEM_VM_CREATE. See struct
>> +     * drm_i915_gem_vm_control.vm_id.
>> +     */
>> +    __u32 vm_id;
>> +    /** @pad: MBZ. */
>> +    __u32 pad;
>> +    /**
>> +     * @offset: The corresponding ppGTT address of the vma which the 
>> kernel
>> +     * will use to perform the lookup.
>> +     */
>> +    __u64 offset;
>> +    /**
>> +     * @attributes: The returned attributes for the given vma.
>> +     *
>> +     * Possible values:
>> +     *
>> +     * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages backing 
>> the
>> +     * vma are currently CPU accessible. If this is not set then the 
>> vma is
>> +     * currently backed by I915_MEMORY_CLASS_DEVICE memory, which the 
>> CPU
>> +     * cannot directly access(this is only possible on discrete 
>> devices with
>> +     * a small BAR). Attempting to MMAP and fault such an object will
>> +     * require the kernel first synchronising any GPU work tied to the
>> +     * object, before then migrating the pages, either to the CPU 
>> accessible
>> +     * part of I915_MEMORY_CLASS_DEVICE, or I915_MEMORY_CLASS_SYSTEM, 
>> if the
>> +     * placements permit it. See 
>> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
>> +     *
>> +     * Note that this is inherently racy.
>> +     */
>> +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
>> +    __u64 attributes;
>> +    /** @rsvd: MBZ */
>> +    __u32 rsvd[4];
>> +};
>> diff --git a/Documentation/gpu/rfc/i915_small_bar.rst 
>> b/Documentation/gpu/rfc/i915_small_bar.rst
>> new file mode 100644
>> index 000000000000..fea92d3d69ab
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
>> @@ -0,0 +1,40 @@
>> +==========================
>> +I915 Small BAR RFC Section
>> +==========================
>> +Starting from DG2 we will have resizable BAR support for device 
>> local-memory,
>> +but in some cases the final BAR size might still be smaller than the 
>> total
>> +local-memory size. In such cases only part of local-memory will be CPU
>> +accessible, while the remainder is only accessible via the GPU.
>> +
>> +I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
>> +----------------------------------------------
>> +New gem_create_ext flag to tell the kernel that a BO will require CPU 
>> access.
>> +The becomes important when placing an object in LMEM, where 
>> underneath the
>> +device has a small BAR, meaning only part of it is CPU accessible. 
>> Without this
>> +flag the kernel will assume that CPU access is not required, and 
>> prioritize
>> +using the non-CPU visible portion of LMEM(if present on the device).
>> +
>> +Related to this, we now also reject any objects marked with
>> +EXEC_OBJECT_CAPTURE, which are also not tagged with NEEDS_CPU_ACCESS. 
>> This only
>> +impacts DG2+.
>> +
>> +XXX: One open here is whether we should extend the memory region 
>> query to return
>> +the CPU visible size of the region. For now the IGTs just use debugfs 
>> to query
>> +the size. However, if userspace sees a real need for this then 
>> extending the
>> +region query would be a lot nicer.
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>> +   :functions: __drm_i915_gem_create_ext
>> +
>> +DRM_I915_QUERY_VMA_INFO query
>> +-----------------------------
>> +Query the attributes of some vma. Given a vm and GTT offset, find the
>> +respective vma, and return its set of attrubutes. For now we only 
>> support
>> +DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the object/vma is
>> +currently placed in memory that is accessible by the CPU. This should 
>> always be
>> +set on devices where the CPU visible size of LMEM matches the probed 
>> size. If
>> +this is not set then CPU faulting the object will first require 
>> migrating the
>> +pages.
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>> +   :functions: __drm_i915_query_vma_info
>> diff --git a/Documentation/gpu/rfc/index.rst 
>> b/Documentation/gpu/rfc/index.rst
>> index 018a8bf317a6..5b8495bdc1fd 100644
>> --- a/Documentation/gpu/rfc/index.rst
>> +++ b/Documentation/gpu/rfc/index.rst
>> @@ -19,3 +19,7 @@ host such documentation:
>>   .. toctree::
>>       i915_scheduler.rst
>> +
>> +.. toctree::
>> +
>> +    i915_small_bar.rst
> 
>

Patch

diff --git a/Documentation/gpu/rfc/i915_small_bar.h b/Documentation/gpu/rfc/i915_small_bar.h
new file mode 100644
index 000000000000..fa65835fd608
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.h
@@ -0,0 +1,153 @@ 
+/**
+ * struct __drm_i915_gem_create_ext - Existing gem_create behaviour, with added
+ * extension support using struct i915_user_extension.
+ *
+ * Note that in the future we want to have our buffer flags here, at least for
+ * the stuff that is immutable. Previously we would have two ioctls, one to
+ * create the object with gem_create, and another to apply various parameters,
+ * however this creates some ambiguity for the params which are considered
+ * immutable. Also in general we're phasing out the various SET/GET ioctls.
+ */
+struct __drm_i915_gem_create_ext {
+	/**
+	 * @size: Requested size for the object.
+	 *
+	 * The (page-aligned) allocated size for the object will be returned.
+	 *
+	 * Note that for some devices we have might have further minimum
+	 * page-size restrictions(larger than 4K), like for device local-memory.
+	 * However in general the final size here should always reflect any
+	 * rounding up, if for example using the I915_GEM_CREATE_EXT_MEMORY_REGIONS
+	 * extension to place the object in device local-memory.
+	 */
+	__u64 size;
+	/**
+	 * @handle: Returned handle for the object.
+	 *
+	 * Object handles are nonzero.
+	 */
+	__u32 handle;
+	/**
+	 * @flags: Optional flags.
+	 *
+	 * Supported values:
+	 *
+	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the kernel that
+	 * the object will need to be accessed via the CPU.
+	 *
+	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE, and
+	 * only strictly required on platforms where only some of the device
+	 * memory is directly visible or mappable through the CPU, like on DG2+.
+	 *
+	 * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
+	 * ensure we can always spill the allocation to system memory, if we
+	 * can't place the object in the mappable part of
+	 * I915_MEMORY_CLASS_DEVICE.
+	 *
+	 * Note that buffers that need to be captured with EXEC_OBJECT_CAPTURE,
+	 * will need to enable this hint, if the object can also be placed in
+	 * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call will
+	 * throw an error otherwise. This also means that such objects will need
+	 * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
+	 *
+	 * Without this hint, the kernel will assume that non-mappable
+	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that the
+	 * kernel can still migrate the object to the mappable part, as a last
+	 * resort, if userspace ever CPU faults this object, but this might be
+	 * expensive, and so ideally should be avoided.
+	 */
+#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
+	__u32 flags;
+	/**
+	 * @extensions: The chain of extensions to apply to this object.
+	 *
+	 * This will be useful in the future when we need to support several
+	 * different extensions, and we need to apply more than one when
+	 * creating the object. See struct i915_user_extension.
+	 *
+	 * If we don't supply any extensions then we get the same old gem_create
+	 * behaviour.
+	 *
+	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
+	 * struct drm_i915_gem_create_ext_memory_regions.
+	 *
+	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
+	 * struct drm_i915_gem_create_ext_protected_content.
+	 */
+#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0
+#define I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
+	__u64 extensions;
+};
+
+#define DRM_I915_QUERY_VMA_INFO	5
+
+/**
+ * struct __drm_i915_query_vma_info
+ *
+ * Given a vm and GTT address, lookup the corresponding vma, returning its set
+ * of attributes.
+ *
+ * .. code-block:: C
+ *
+ *	struct drm_i915_query_vma_info info = {};
+ *	struct drm_i915_query_item item = {
+ *		.data_ptr = (uintptr_t)&info,
+ *		.query_id = DRM_I915_QUERY_VMA_INFO,
+ *	};
+ *	struct drm_i915_query query = {
+ *		.num_items = 1,
+ *		.items_ptr = (uintptr_t)&item,
+ *	};
+ *	int err;
+ *
+ *	// Unlike some other types of queries, there is no need to first query
+ *	// the size of the data_ptr blob here, since we already know ahead of
+ *	// time how big this needs to be.
+ *	item.length = sizeof(info);
+ *
+ *	// Next we fill in the vm_id and ppGTT address of the vma we wish
+ *	// to query, before then firing off the query.
+ *	info.vm_id = vm_id;
+ *	info.offset = gtt_address;
+ *	err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
+ *	if (err || item.length < 0) ...
+ *
+ *	// If all went well we can now inspect the returned attributes.
+ *	if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
+ */
+struct __drm_i915_query_vma_info {
+	/**
+	 * @vm_id: The given vm id that contains the vma. The id is the value
+	 * returned by the DRM_I915_GEM_VM_CREATE. See struct
+	 * drm_i915_gem_vm_control.vm_id.
+	 */
+	__u32 vm_id;
+	/** @pad: MBZ. */
+	__u32 pad;
+	/**
+	 * @offset: The corresponding ppGTT address of the vma which the kernel
+	 * will use to perform the lookup.
+	 */
+	__u64 offset;
+	/**
+	 * @attributes: The returned attributes for the given vma.
+	 *
+	 * Possible values:
+	 *
+	 * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set if the pages backing the
+	 * vma are currently CPU accessible. If this is not set then the vma is
+	 * currently backed by I915_MEMORY_CLASS_DEVICE memory, which the CPU
+	 * cannot directly access(this is only possible on discrete devices with
+	 * a small BAR). Attempting to MMAP and fault such an object will
+	 * require the kernel first synchronising any GPU work tied to the
+	 * object, before then migrating the pages, either to the CPU accessible
+	 * part of I915_MEMORY_CLASS_DEVICE, or I915_MEMORY_CLASS_SYSTEM, if the
+	 * placements permit it. See I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
+	 *
+	 * Note that this is inherently racy.
+	 */
+#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
+	__u64 attributes;
+	/** @rsvd: MBZ */
+	__u32 rsvd[4];
+};
diff --git a/Documentation/gpu/rfc/i915_small_bar.rst b/Documentation/gpu/rfc/i915_small_bar.rst
new file mode 100644
index 000000000000..fea92d3d69ab
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.rst
@@ -0,0 +1,40 @@ 
+==========================
+I915 Small BAR RFC Section
+==========================
+Starting from DG2 we will have resizable BAR support for device local-memory,
+but in some cases the final BAR size might still be smaller than the total
+local-memory size. In such cases only part of local-memory will be CPU
+accessible, while the remainder is only accessible via the GPU.
+
+I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
+----------------------------------------------
+New gem_create_ext flag to tell the kernel that a BO will require CPU access.
+The becomes important when placing an object in LMEM, where underneath the
+device has a small BAR, meaning only part of it is CPU accessible. Without this
+flag the kernel will assume that CPU access is not required, and prioritize
+using the non-CPU visible portion of LMEM(if present on the device).
+
+Related to this, we now also reject any objects marked with
+EXEC_OBJECT_CAPTURE, which are also not tagged with NEEDS_CPU_ACCESS. This only
+impacts DG2+.
+
+XXX: One open here is whether we should extend the memory region query to return
+the CPU visible size of the region. For now the IGTs just use debugfs to query
+the size. However, if userspace sees a real need for this then extending the
+region query would be a lot nicer.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
+   :functions: __drm_i915_gem_create_ext
+
+DRM_I915_QUERY_VMA_INFO query
+-----------------------------
+Query the attributes of some vma. Given a vm and GTT offset, find the
+respective vma, and return its set of attrubutes. For now we only support
+DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the object/vma is
+currently placed in memory that is accessible by the CPU. This should always be
+set on devices where the CPU visible size of LMEM matches the probed size. If
+this is not set then CPU faulting the object will first require migrating the
+pages.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
+   :functions: __drm_i915_query_vma_info
diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
index 018a8bf317a6..5b8495bdc1fd 100644
--- a/Documentation/gpu/rfc/index.rst
+++ b/Documentation/gpu/rfc/index.rst
@@ -19,3 +19,7 @@  host such documentation:
 .. toctree::
 
     i915_scheduler.rst
+
+.. toctree::
+
+    i915_small_bar.rst