[Intel-gfx] [PATCH v2] drm/doc: add rfc section for small BAR uapi
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Wed Apr 27 08:36:49 UTC 2022
On 20/04/2022 18:13, 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;
Would unallocated_cpu_visible_size be useful, to follow the total
unallocated_size?
Btw, have we ever considered whether unallocated_size should require
CAP_SYS_ADMIN/PERFMON or something?
> + };
> + };
> +};
> +
> +/**
> + * 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.
> + */
So "needs_cpu_access" flag could almost be viewed as a sub-region
placement priority? What I mean is this:
1)
placements=device,system flags=
This results in placement priorities: device, device_cpu_mappable, system.
2)
placements=device,system flags=needs_cpu_access
This results in placement priorities: device_cpu_mappable, device, system.
Is this correct?
The benefit of the flag is that i915 can place the object to the right
place from the start instead of on the first CPU access? Is that worth
it or is there more to it?
> +#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.
I think there should be justification for the new query documented as
well. (Why on top of what.)
Without it personally I can't immediately understand why the disconnect
between the object based and VMA based API. Userspace has to do some
intervening operations like either execbuf, or vm bind in the future, to
make this query usable after object creation. So question is why
wouldn't it know already which placements it allowed and so would i915
auto-migrate or not for this particular object. No? Or in other words
why this wouldn't be an object based query since the question it is
answering is about the object backing store and not the VMA.
Regards,
Tvrtko
> +
> +.. 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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