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

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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 <lionel.g.landwerlin@xxxxxxxxx>
Sent: Monday, May 2, 2022 12:55 AM
To: Auld, Matthew <matthew.auld@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx; Thomas Hellström
<thomas.hellstrom@xxxxxxxxxxxxxxx>; Bloomfield, Jon
<jon.bloomfield@xxxxxxxxx>; Daniel Vetter <daniel.vetter@xxxxxxxx>; Justen,
Jordan L <jordan.l.justen@xxxxxxxxx>; Kenneth Graunke
<kenneth@xxxxxxxxxxxxx>; Abodunrin, Akeem G
<akeem.g.abodunrin@xxxxxxxxx>; mesa-dev@xxxxxxxxxxxxxxxxxxxxx
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 <matthew.auld@xxxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Jordan Justen <jordan.l.justen@xxxxxxxxx>
Cc: Kenneth Graunke <kenneth@xxxxxxxxxxxxx>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@xxxxxxxxx>
Cc: mesa-dev@xxxxxxxxxxxxxxxxxxxxx
---
   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,


-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





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