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

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?

What value is returned when all of probed_size is CPU visible on local 
memory?


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