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

On 03/05/2022 11:39, Lionel Landwerlin wrote:
> 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.

So do we want this in the next version? i.e we already have a current 
real use case in mind for unallocated_size where probed_size is not good 
enough?

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