[PATCH v2] drm/doc: add rfc section for small BAR uapi
Lionel Landwerlin
lionel.g.landwerlin at intel.com
Wed Apr 27 15:37:39 UTC 2022
On 27/04/2022 18:18, Matthew Auld wrote:
> On 27/04/2022 07:48, Lionel Landwerlin wrote:
>> One question though, how do we detect that this flag
>> (I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS) is accepted on a given
>> kernel?
>> I assume older kernels are going to reject object creation if we use
>> this flag?
>
> From some offline discussion with Lionel, the plan here is to just do
> a dummy gem_create_ext to check if the kernel throws an error with the
> new flag or not.
>
>>
>> I didn't plan to use __drm_i915_query_vma_info, but isn't it
>> inconsistent to select the placement on the GEM object and then query
>> whether it's mappable by address?
>> You made a comment stating this is racy, wouldn't querying on the GEM
>> object prevent this?
>
> Since mesa at this time doesn't currently have a use for this one,
> then I guess we should maybe just drop this part of the uapi, in this
> version at least, if no objections.
Just repeating what we discussed (maybe I missed some other discussion
and that's why I was confused) :
The way I was planning to use this is to have 3 heaps in Vulkan :
- heap0: local only, no cpu visible
- heap1: system, cpu visible
- heap2: local & cpu visible
With heap2 having the reported probed_cpu_visible_size size.
It is an error for the application to map from heap0 [1].
With that said, it means if we created a GEM BO without
I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS, we'll never mmap it.
So why the query?
I guess it would be useful when we import a buffer from another
application. But in that case, why not have the query on the BO?
-Lionel
[1] :
https://www.khronos.org/registry/vulkan/specs/1.3-extensions/man/html/vkMapMemory.html
(VUID-vkMapMemory-memory-00682)
>
>>
>> Thanks,
>>
>> -Lionel
>>
>> On 27/04/2022 09:35, Lionel Landwerlin wrote:
>>> Hi Matt,
>>>
>>>
>>> The proposal looks good to me.
>>>
>>> Looking forward to try it on drm-tip.
>>>
>>>
>>> -Lionel
>>>
>>> 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 <matthew.auld at intel.com>
>>>> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>>> Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>>>> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
>>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>>> Cc: Jordan Justen <jordan.l.justen at intel.com>
>>>> Cc: Kenneth Graunke <kenneth at whitecape.org>
>>>> Cc: Akeem G Abodunrin <akeem.g.abodunrin at intel.com>
>>>> Cc: mesa-dev at lists.freedesktop.org
>>>> ---
>>>> 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;
>>>> + };
>>>> + };
>>>> +};
>>>> +
>>>> +/**
>>>> + * 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
>>>
>>>
>>
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