[Intel-gfx] [PATCH v2] drm/doc: add rfc section for small BAR uapi
Lionel Landwerlin
lionel.g.landwerlin at intel.com
Wed Apr 27 06:48:45 UTC 2022
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?
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?
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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