Re: [PATCH v2] drm/doc: add rfc section for small BAR uapi

On 03/05/2022 12:07, Matthew Auld wrote:
> On 02/05/2022 19:03, Lionel Landwerlin wrote:
>> On 02/05/2022 20:58, Abodunrin, Akeem G wrote:
>>>
>>>> -----Original Message-----
>>>> From: Landwerlin, Lionel G <[email protected]>
>>>> Sent: Monday, May 2, 2022 12:55 AM
>>>> To: Auld, Matthew <[email protected]>; 
>>>> [email protected]
>>>> Cc: [email protected]; Thomas Hellström
>>>> <[email protected]>; Bloomfield, Jon
>>>> <[email protected]>; Daniel Vetter <[email protected]>; 
>>>> Justen,
>>>> Jordan L <[email protected]>; Kenneth Graunke
>>>> <[email protected]>; Abodunrin, Akeem G
>>>> <[email protected]>; [email protected]
>>>> Subject: Re: [PATCH v2] drm/doc: add rfc section for small BAR uapi
>>>>
>>>> On 20/04/2022 20:13, Matthew Auld wrote:
>>>>> Add an entry for the new uapi needed for small BAR on DG2+.
>>>>>
>>>>> v2:
>>>>>     - Some spelling fixes and other small tweaks. (Akeem & Thomas)
>>>>>     - Rework error capture interactions, including no longer needing
>>>>>       NEEDS_CPU_ACCESS for objects marked for capture. (Thomas)
>>>>>     - Add probed_cpu_visible_size. (Lionel)
>>>>>
>>>>> Signed-off-by: Matthew Auld <[email protected]>
>>>>> Cc: Thomas Hellström <[email protected]>
>>>>> Cc: Lionel Landwerlin <[email protected]>
>>>>> Cc: Jon Bloomfield <[email protected]>
>>>>> Cc: Daniel Vetter <[email protected]>
>>>>> Cc: Jordan Justen <[email protected]>
>>>>> Cc: Kenneth Graunke <[email protected]>
>>>>> Cc: Akeem G Abodunrin <[email protected]>
>>>>> Cc: [email protected]
>>>>> ---
>>>>>    Documentation/gpu/rfc/i915_small_bar.h   | 190
>>>> +++++++++++++++++++++++
>>>>> Documentation/gpu/rfc/i915_small_bar.rst |  58 +++++++
>>>>>    Documentation/gpu/rfc/index.rst          |   4 +
>>>>>    3 files changed, 252 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..7bfd0cf44d35
>>>>> --- /dev/null
>>>>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>>>>> @@ -0,0 +1,190 @@
>>>>> +/**
>>>>> + * struct __drm_i915_memory_region_info - Describes one region as
>>>>> +known to the
>>>>> + * driver.
>>>>> + *
>>>>> + * Note this is using both struct drm_i915_query_item and struct
>>>> drm_i915_query.
>>>>> + * For this new query we are adding the new query id
>>>>> +DRM_I915_QUERY_MEMORY_REGIONS
>>>>> + * at &drm_i915_query_item.query_id.
>>>>> + */
>>>>> +struct __drm_i915_memory_region_info {
>>>>> +   /** @region: The class:instance pair encoding */
>>>>> +   struct drm_i915_gem_memory_class_instance region;
>>>>> +
>>>>> +   /** @rsvd0: MBZ */
>>>>> +   __u32 rsvd0;
>>>>> +
>>>>> +   /** @probed_size: Memory probed by the driver (-1 = unknown) */
>>>>> +   __u64 probed_size;
>>>>> +
>>>>> +   /** @unallocated_size: Estimate of memory remaining (-1 = 
>>>>> unknown)
>>>> */
>>>>> +   __u64 unallocated_size;
>>>>> +
>>>>> +   union {
>>>>> +           /** @rsvd1: MBZ */
>>>>> +           __u64 rsvd1[8];
>>>>> +           struct {
>>>>> +                   /**
>>>>> +                    * @probed_cpu_visible_size: Memory probed by the
>>>> driver
>>>>> +                    * that is CPU accessible. (-1 = unknown).
>>>>> +                    *
>>>>> +                    * This will be always be <= @probed_size, and 
>>>>> the
>>>>> +                    * remainder(if there is any) will not be CPU
>>>>> +                    * accessible.
>>>>> +                    */
>>>>> +                   __u64 probed_cpu_visible_size;
>>>>> +           };
>>>>
>>>> Trying to implement userspace support in Vulkan for this, I have an 
>>>> additional
>>>> question about the value of probed_cpu_visible_size.
>>>>
>>>> When is it set to -1?
>>> I believe it is set to -1 if it is unknown, and/or not cpu 
>>> accessible...
>>>
>>> Cheers!
>>> ~Akeem
>>
>>
>> So what should I expect on system memory?
>
> I guess just probed_cpu_visible_size == probed_size. Or maybe we can 
> just use -1 here?
>
>>
>> What value is returned when all of probed_size is CPU visible on 
>> local memory?
>
> probed_size == probed_cpu_visible_size.


Thanks, looks good to me.

Then maybe we should update the comment to say that.

Looks like there are no cases where we'll get -1.


-Lionel


>
>>
>>
>> Thanks,
>>
>>
>> -Lionel
>>
>>
>>>> I'm guessing before there is support for this value it'll be 0 (MBZ).
>>>>
>>>> After after it should either be the entire lmem or something smaller.
>>>>
>>>>
>>>> -Lionel
>>>>
>>>>
>>>>> +   };
>>>>> +};
>>>>> +
>>>>> +/**
>>>>> + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour,
>>>>> +with added
>>>>> + * extension support using struct i915_user_extension.
>>>>> + *
>>>>> + * Note that new buffer flags should be added 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 since the kernel only supports flat-CCS on 
>>>>> objects that can
>>>>> +    * *only* be placed in I915_MEMORY_CLASS_DEVICE, we therefore
>>>> don't
>>>>> +    * support I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS
>>>> together with
>>>>> +    * flat-CCS.
>>>>> +    *
>>>>> +    * 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..be3d9bcdd86d
>>>>> --- /dev/null
>>>>> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
>>>>> @@ -0,0 +1,58 @@
>>>>> +==========================
>>>>> +I915 Small BAR RFC Section
>>>>> +==========================
>>>>> +Starting from DG2 we will have resizable BAR support for device
>>>>> +local-memory(i.e I915_MEMORY_CLASS_DEVICE), but in some cases the
>>>>> +final BAR size might still be smaller than the total probed_size. In
>>>>> +such cases, only some subset of I915_MEMORY_CLASS_DEVICE will be CPU
>>>>> +accessible(for example the first 256M), 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.
>>>>> +This becomes important when placing an object in
>>>>> +I915_MEMORY_CLASS_DEVICE, where underneath the device has a small
>>>>> +BAR, meaning only some portion 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
>>>> I915_MEMORY_CLASS_DEVICE.
>>>>> +
>>>>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>>>>> +   :functions: __drm_i915_gem_create_ext
>>>>> +
>>>>> +probed_cpu_visible_size attribute
>>>>> +---------------------------------
>>>>> +New struct__drm_i915_memory_region attribute which returns the total
>>>>> +size of the CPU accessible portion, for the particular region. This
>>>>> +should only be applicable for I915_MEMORY_CLASS_DEVICE.
>>>>> +
>>>>> +Vulkan will need this as part of creating a separate VkMemoryHeap
>>>>> +with the VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT set, to represent the
>>>>> +CPU visible portion, where the total size of the heap needs to be 
>>>>> known.
>>>>> +
>>>>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>>>>> +   :functions: __drm_i915_memory_region_info
>>>>> +
>>>>> +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 attributes. 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
>>>>> +probed_cpu_visible_size of I915_MEMORY_CLASS_DEVICE matches the
>>>>> +probed_size. If this is not set then CPU faulting the object will 
>>>>> likely first
>>>> require migrating the pages.
>>>>> +
>>>>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>>>>> +   :functions: __drm_i915_query_vma_info
>>>>> +
>>>>> +Error Capture restrictions
>>>>> +--------------------------
>>>>> +With error capture we have two new restrictions:
>>>>> +
>>>>> +    1) Error capture is best effort on small BAR systems; if the 
>>>>> pages are not
>>>>> +    CPU accessible, at the time of capture, then the kernel is 
>>>>> free to skip
>>>>> +    trying to capture them.
>>>>> +
>>>>> +    2) On discrete we now reject error capture on recoverable 
>>>>> contexts. In the
>>>>> +    future the kernel may want to blit during error capture, when 
>>>>> for example
>>>>> +    something is not currently CPU accessible.
>>>>> diff --git a/Documentation/gpu/rfc/index.rst
>>>>> b/Documentation/gpu/rfc/index.rst index 91e93a705230..5a3bd3924ba6
>>>>> 100644
>>>>> --- a/Documentation/gpu/rfc/index.rst
>>>>> +++ b/Documentation/gpu/rfc/index.rst
>>>>> @@ -23,3 +23,7 @@ host such documentation:
>>>>>    .. toctree::
>>>>>
>>>>>        i915_scheduler.rst
>>>>> +
>>>>> +.. toctree::
>>>>> +
>>>>> +    i915_small_bar.rst
>>
>>