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

On 03/05/2022 13:22, Matthew Auld wrote:
> On 02/05/2022 09:53, Lionel Landwerlin wrote:
>> On 02/05/2022 10:54, Lionel Landwerlin wrote:
>>> 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 <[email protected]>
>>>> Cc: Thomas Hellström <[email protected]>
>>>> Cc: Lionel Landwerlin <[email protected]>
>>>> Cc: Jon Bloomfield <[email protected]>
>>>> Cc: Daniel Vetter <[email protected]>
>>>> Cc: Jordan Justen <[email protected]>
>>>> Cc: Kenneth Graunke <[email protected]>
>>>> Cc: Akeem G Abodunrin <[email protected]>
>>>> Cc: [email protected]
>>>> ---
>>>>   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'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
>>
>>
>> Other pain point of this new uAPI, previously we could query the 
>> unallocated size for each heap.
>
> unallocated_size should always give the same value as probed_size. We 
> have the avail tracking, but we don't currently expose that through 
> unallocated_size, due to lack of real userspace/user etc.
>
>>
>> Now lmem is effectively divided into 2 heaps, but unallocated_size is 
>> tracking allocation from both parts of lmem.
>
> Yeah, if we ever properly expose the unallocated_size, then we could 
> also just add unallocated_cpu_visible_size.
>
>>
>> Is adding new I915_MEMORY_CLASS_DEVICE_NON_MAPPABLE out of question?
>
> I don't think it's out of the question...
>
> I guess user-space should be able to get the current flag behaviour 
> just by specifying: device, system. And it does give more flexibly to 
> allow something like: device, device-nm, smem.
>
> We can also drop the probed_cpu_visible_size, which would now just be 
> the probed_size with device/device-nm. And if we lack device-nm, then 
> the entire thing must be CPU mappable.
>
> One of the downsides though, is that we can no longer easily mix 
> object pages from both device + device-nm, which we could previously 
> do when we didn't specify the flag. At least according to the current 
> design/behaviour for @regions that would not be allowed. I guess some 
> kind of new flag like ALLOC_MIXED or so? Although currently that is 
> only possible with device + device-nm in ttm/i915.


Thanks, I wasn't aware of the restrictions.

Adding unallocated_cpu_visible_size would be great.


-Lionel


>
>>
>>
>> -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
>>>
>>>
>>