Re: [PATCH v3] drm/doc: add rfc section for small BAR uapi

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On 17/05/2022 10:12, Matthew Auld wrote:
On 17/05/2022 09:29, Tvrtko Ursulin wrote:

On 16/05/2022 19:11, 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)
v3:
   - Drop the vma query for now.
   - Add unallocated_cpu_visible_size as part of the region query.
   - Improve the docs some more, including documenting the expected
     behaviour on older kernels, since this came up in some offline
     discussion.

Signed-off-by: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Jon Bloomfield <jon.bloomfield@xxxxxxxxx>
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   | 164 +++++++++++++++++++++++
  Documentation/gpu/rfc/i915_small_bar.rst |  47 +++++++
  Documentation/gpu/rfc/index.rst          |   4 +
  3 files changed, 215 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..4079d287750b
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.h
@@ -0,0 +1,164 @@
+/**
+ * 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;

Is -1 possible today or when it will be? For system memory it appears zeroes are returned today so that has to stay I think. Does it effectively mean userspace has to consider both 0 and -1 as unknown is the question.

It looks like it just returns the totalram_pages(). So at the moment nothing ever currently returns -1 or 0. Maybe that was a mistake for I915_MEMORY_SYSTEM.

Yes I figured out my confusion later.


+
+    /**
+     * @unallocated_size: Estimate of memory remaining (-1 = unknown)
+     *
+     * Note this is only currently tracked for I915_MEMORY_CLASS_DEVICE
+     * regions, and also requires CAP_PERFMON or CAP_SYS_ADMIN to get
+     * reliable accounting. Without this(or if this an older kernel) the

s/if this an/if this is an/

Also same question as above about -1.

This should be the same as above, since this will give the same value as probed_size, or give the real avail value for lmem.


+     * value here will always match the @probed_size.
+     */
+    __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).

Also question about -1. In this case this could be done since the field is yet to be added but I am curious if it ever can be -1.

I was just going to make this the same as probed_size for system memory. But we could use -1 here instead. What do you think? Same for unallocated below.

I don't completely follow the instead part, when you already have documented it?

-1 will start appearing with changes that require CAP_SYS_PERFMON right?

Regards,

Tvrtko

+             *
+             * This will be always be <= @probed_size, and the
+             * remainder(if there is any) will not be CPU
+             * accessible.
+             *
+             * On systems without small BAR, the @probed_size will
+             * always equal the @probed_cpu_visible_size, since all
+             * of it will be CPU accessible.
+             *
+             * Note that if the value returned here is zero, then
+             * this must be an old kernel which lacks the relevant
+             * small-bar uAPI support(including

I have noticed you prefer no space before parentheses throughout the text so I guess it's just my preference to have it. Very nitpicky even if I am right so up to you.

Ok, will change :)


+             * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS), but on
+             * such systems we should never actually end up with a
+             * small BAR configuration, assuming we are able to load
+             * the kernel module. Hence it should be safe to treat
+             * this the same as when @probed_cpu_visible_size ==
+             * @probed_size.
+             */
+            __u64 probed_cpu_visible_size;
+
+            /**
+             * @unallocated_cpu_visible_size: Estimate of CPU
+             * visible memory remaining (-1 = unknown).
+             *
+             * Note this is only currently tracked for
+             * I915_MEMORY_CLASS_DEVICE regions, and also requires
+             * CAP_PERFMON or CAP_SYS_ADMIN to get reliable
+             * accounting. Without this the value here will always
+             * equal the @probed_cpu_visible_size.
+             */
+            __u64 unallocated_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.

Is it defined how rounding up works when a list of regions is given (like smem+lmem) and should that be explicitly mentioned here?

Yes, it uses the largest minimum page-size for the set of regions when doing the round_up. Will tweak.


+     */
+    __u64 size;

Blank line here (etc below) maybe to match the previous doc block?

Ok, will tweak. Thanks for the comments and suggestions.


Regards,

Tvrtko

+    /**
+     * @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 configurations where some subset of the device +     * memory is directly visible/mappable through the CPU(which we also +     * call small BAR), like on some DG2+ systems. Note that this is quite +     * undesirable, but due to various factors like the client CPU, BIOS etc
+     * it's something we can expect to see in the wild. See struct
+     * __drm_i915_memory_region_info.probed_cpu_visible_size for how to
+     * determine if this system applies.
+     *
+     * Note that one of the placements MUST be I915_MEMORY_CLASS_SYSTEM, to +     * ensure the kernel can always spill the allocation to system memory,
+     * if the object can't be allocated in the mappable part of
+     * I915_MEMORY_CLASS_DEVICE.
+     *
+     * Also 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.
+     *
+     * On older kernels, where usage of this flag results in an error, since
+     * we lack the relevant small BAR uAPI(see also struct
+     * __drm_i915_memory_region_info.probed_cpu_visible_size) it should
+     * NEVER be possible to end up with a small BAR configuration, assuming +     * we can also successfully load the i915 kernel module. In such cases +     * the entire I915_MEMORY_CLASS_DEVICE region will be CPU accessible, +     * and as such there are zero restrictions on where the object can be
+     * placed.
+     */
+#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;
+};
diff --git a/Documentation/gpu/rfc/i915_small_bar.rst b/Documentation/gpu/rfc/i915_small_bar.rst
new file mode 100644
index 000000000000..a322481cea8b
--- /dev/null
+++ b/Documentation/gpu/rfc/i915_small_bar.rst
@@ -0,0 +1,47 @@
+==========================
+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. We also report the
+unallocated_cpu_visible_size, alongside the unallocated_size.
+
+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. It also wants to be able to
+give a rough estimate of how memory can potentially be allocated.
+
+.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
+   :functions: __drm_i915_memory_region_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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