Re: [Intel-gfx] [PATCH 2/2] drm/doc: add rfc section for small BAR uapi

Matthew Auld
2022-03-18T10:21:41Z

On 18/03/2022 09:38, Lionel Landwerlin wrote:
> Hey Matthew, all,
>
> This sounds like a good thing to have.
> There are a number of DG2 machines where we have a small BAR and this is
> causing more apps to fail.
>
> Anv currently reports 3 memory heaps to the app :
>
>     - local device only (not host visible) -> mapped to lmem
>     - device/cpu -> mapped to smem
>     - local device but also host visible -> mapped to lmem
>
> So we could use this straight away, by just not putting the
> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag on the allocation of the
> first heap.
>
> One thing I don't see in this proposal is how can we get the size of the
> 2 lmem heap : cpu visible, cpu not visible
> We could use that to report the appropriate size to the app.
> We probably want to report a new drm_i915_memory_region_info and either :
>     - put one of the reserve field to use to indicate : cpu visible
>     - or define a new enum value in drm_i915_gem_memory_class

Thanks for taking a look at this. Returning the probed CPU visible size
as part of the region query seems reasonable. Something like:

@@ -3074,8 +3074,18 @@ struct drm_i915_memory_region_info {
/** @unallocated_size: Estimate of memory remaining (-1 =
unknown) */
__u64 unallocated_size;

- /** @rsvd1: MBZ */
- __u64 rsvd1[8];
+ union {
+ /** @rsvd1: MBZ */
+ __u64 rsvd1[8];
+
+ struct {
+ /**
+ * @probed_cpu_visible_size: Memory probed by
the driver
+ * that is CPU accessible. (-1 = unknown)
+ */
+ __u64 probed_cpu_visible_size;
+ };
+ };

I will add this in the next version, if no objections.

>
> Cheers,
>
> -Lionel
>
>
> On 18/02/2022 13:22, Matthew Auld wrote:
>> Add an entry for the new uapi needed for small BAR on DG2+.
>>
>> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>> Cc: Jon Bloomfield <jon.bloomfield@intel.com>
>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Kenneth Graunke <kenneth@whitecape.org>
>> Cc: mesa-dev@lists.freedesktop.org
>> ---
>>   Documentation/gpu/rfc/i915_small_bar.h   | 153 +++++++++++++++++++++++
>>   Documentation/gpu/rfc/i915_small_bar.rst |  40 ++++++
>>   Documentation/gpu/rfc/index.rst          |   4 +
>>   3 files changed, 197 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..fa65835fd608
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>> @@ -0,0 +1,153 @@
>> +/**
>> + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour,
>> with added
>> + * extension support using struct i915_user_extension.
>> + *
>> + * Note that in the future we want to have our buffer flags 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 buffers that need to be captured with
>> EXEC_OBJECT_CAPTURE,
>> +     * will need to enable this hint, if the object can also be
>> placed in
>> +     * I915_MEMORY_CLASS_DEVICE, starting from DG2+. The execbuf call
>> will
>> +     * throw an error otherwise. This also means that such objects
>> will need
>> +     * I915_MEMORY_CLASS_SYSTEM set as a possible placement.
>> +     *
>> +     * 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..fea92d3d69ab
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
>> @@ -0,0 +1,40 @@
>> +==========================
>> +I915 Small BAR RFC Section
>> +==========================
>> +Starting from DG2 we will have resizable BAR support for device
>> local-memory,
>> +but in some cases the final BAR size might still be smaller than the
>> total
>> +local-memory size. In such cases only part of local-memory will be CPU
>> +accessible, 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.
>> +The becomes important when placing an object in LMEM, where
>> underneath the
>> +device has a small BAR, meaning only part 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 LMEM(if present on the device).
>> +
>> +Related to this, we now also reject any objects marked with
>> +EXEC_OBJECT_CAPTURE, which are also not tagged with NEEDS_CPU_ACCESS.
>> This only
>> +impacts DG2+.
>> +
>> +XXX: One open here is whether we should extend the memory region
>> query to return
>> +the CPU visible size of the region. For now the IGTs just use debugfs
>> to query
>> +the size. However, if userspace sees a real need for this then
>> extending the
>> +region query would be a lot nicer.
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>> +   :functions: __drm_i915_gem_create_ext
>> +
>> +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 attrubutes. 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 visible size of LMEM matches the probed
>> size. If
>> +this is not set then CPU faulting the object will first require
>> migrating the
>> +pages.
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>> +   :functions: __drm_i915_query_vma_info
>> diff --git a/Documentation/gpu/rfc/index.rst
>> b/Documentation/gpu/rfc/index.rst
>> index 018a8bf317a6..5b8495bdc1fd 100644
>> --- a/Documentation/gpu/rfc/index.rst
>> +++ b/Documentation/gpu/rfc/index.rst
>> @@ -19,3 +19,7 @@ host such documentation:
>>   .. toctree::
>>       i915_scheduler.rst
>> +
>> +.. toctree::
>> +
>> +    i915_small_bar.rst
>
>