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

On 03/05/2022 17:27, Matthew Auld wrote:
> 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?


Yeah in the  next iteration.

We're using unallocated_size to implement VK_EXT_memory_budget and since 
I'm going to expose lmem mappable/unmappable as 2 different heaps on 
Vulkan, I would use that there too.


-Lionel


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